fix(#4254): pin sequential executor to the orchestrator's validated root (#4476)

* test(#4254): sequential executor root pin — failing-first regression + matrix

The new suite executes the shipped supplied-root-pin guard against real git
fixtures (drifted primary-checkout cwd halts before the write and the FATAL
names both roots; matching cwd permits it; unexpanded/empty pins halt;
normalization forms; submodule and sibling boundaries; metacharacter quoting;
drive-letter form gate) and locks the dispatch contract across execute-phase.md,
its sequential-root-pin step fragment, and worktree-path-safety.md. The #2772
per-plan serialization assertion retargets to the fragment that now carries
those rules (ADR-857 Phase 6 ceiling), plus the host-step wiring.

* fix(#4254): pin sequential executor to the orchestrator's validated root

Sequential-mode dispatch told the executor to self-derive PROJECT_ROOT from its
own cwd; every existing guard is worktree-mode-only or self-referential, so an
executor spawned with a drifted cwd committed onto the wrong checkout silently.

- worktree-path-safety.md step 0p: mode-agnostic supplied-root pin guard,
  composed by the orchestrator at build time with the literal $ORCHESTRATOR_WT
  (git-vs-git comparison on both sides — representation-safe on Windows, the
  #4296 lesson), fail-closed on empty/unexpanded pins, registered-submodule
  allowance, warn-and-proceed only when the dispatch carries no pin block.
- execute-phase.md sequential branch: build-time embed of the bound
  <project_root_pin> via the new execute-phase/steps/sequential-root-pin.md
  fragment (ADR-857 Phase 6 frozen ceiling — the host step cannot grow; the
  wave serialization rules move with the fragment, verbatim in substance) plus
  the per-write/commit pin instruction in <sequential_execution>. Worktree-mode
  dispatch untouched (its self-derived toplevel IS correct there).
- INVENTORY rows (5 locales) + INVENTORY-MANIFEST + install-tree goldens
  regenerated for the new fragment; changeset added.

* chore(#4254): backfill changeset PR number

* fix(#4254): accept backslash-separated Windows drive pins

CI on windows-latest showed every permit-path test failing with
"Actual root: <none>": pins composed from Node's path.join arrive in the
backslash drive form (C:\Users\RUNNER~1\...), which the guard's absolute-form
gate rejected before the cwd-side root was ever computed — a legitimate
matching pin could never pass. The gate now accepts either separator
([A-Za-z]:[\\/]); git -C resolves both forms (and 8.3 short names) to the
same canonical toplevel, so the git-vs-git comparison is unaffected. Form-gate
tests cover the emitted (C:/…) and produced (C:\…) spellings plus short names.

* fix(#4254): portable drive-form gate for MSYS bash

The bracket class [\\/] that accepted backslash drive pins parses
inconsistently on MSYS bash (the Windows CI leg still rejected C:\ pins —
every permit-path test red with "Actual root: <none>"). Replace it with
standard pattern escaping outside brackets: [A-Za-z]:/*|[A-Za-z]:\\* —
the escape form is version- and build-portable. Verified across all forms:
both drive spellings accepted; bare "C:", relative, empty, and unexpanded
rejected.

* fix(#4254): runtime-generated backslash comparator + self-describing FATAL

The Windows CI legs failed every #4254 permit-path row with
'Actual root: <none>' across two prior pattern spellings ([\\/] and \\*).
Stage misattribution: <none> appears whenever the FATAL fires BEFORE the
cwd-side capture assigns ACTUAL_ROOT — the absolute-form gate was what fired.

Mechanism: the test harness spawns bash -c <script> through the Windows
command-line boundary; that round-trip applies one extra shell-quoting pass
with double-quote semantics — a backslash written twice in the script text
arrives halved, while a lone backslash survives (the pin displays intact;
row 9's pure-bash gate independently showed the halved pattern rejecting
C:\ while C:/ still passed its surviving arm). On windows-latest every pin
carries backslashes (os.tmpdir() is the 8.3 short form C:\Users\RUNNER~1\...),
so the gate ate every pin before the actual root was ever computed.

Fix, robust by construction:
- the drive-form gate generates its backslash comparator at RUNTIME
  (BS=$(printf '\134'); match [A-Za-z]:"$BS"*) — the shipped guard now
  contains no doubled backslash anywhere, enforced by a regression
  assertion on the extracted guard text;
- the FATAL self-describes: Guard stage (pin-unbound / form-gate /
  actual-capture / pinned-capture / root-mismatch) plus a Diagnostic line
  carrying git's own stderr for capture failures and both compared values
  for mismatches — future platform failures name their stage in the log;
- row 9's hand-rolled duplicate case gate (transit-fragile copy, #4296
  Minor 1 duplication smell) is replaced by driving the SHIPPED guard and
  asserting the stage; rows 2/4 pin the new stage machinery.

Validated on darwin across drift/match/relative/unbound/empty/bare-drive/
forward-and-backslash drive forms, each also re-run under a simulated
Windows transit (every doubled backslash halved) with identical outcomes.

* fix(#4254): close the empty-comparator fail-open seam in the drive-form gate

Self-review of the runtime-generated backslash comparator: if printf's
octal escape ever returned empty, the drive arm [A-Za-z]:"$BS"* would
widen to drive-RELATIVE pins (C:foo) — the construction's one theoretical
fail-open path. Fail closed with a self-describing diagnostic instead of
trusting the shell's printf.

---------

Co-authored-by: sim <sim@local>

gsd-core/workflows/execute-phase.md

@@ -815,14 +815,23 @@ increases monotonically across waves. `{status}` is `complete` (success),
 
    **Sequential mode** (`USE_WORKTREES_FOR_PLAN` is `false` — either project-level `USE_WORKTREES=false`, or per-plan submodule intersection forced it false in step 2.5):
 
-   Omit `isolation="worktree"` from the Agent call. Replace the `<parallel_execution>` block with:
+   Omit `isolation="worktree"` from the Agent call. Before composing the prompt, read and execute
+   `execute-phase/steps/sequential-root-pin.md` (#4254) — it owns the sequential root-pin build-time
+   embed and the wave serialization rules.
+
+   Replace the `<parallel_execution>` block with:
 
    ```
        <sequential_execution>
        You are running as a SEQUENTIAL executor agent on the main working tree.
        Use normal git commits (with hooks). Do NOT use --no-verify.
+       Run the `<project_root_pin>` guard before your first Edit/Write and before every commit (#4254).
        REQUIRED ORDER: Write SUMMARY.md → commit → only then any narration. No text between Write and commit (truncation risk; #2070 rescue is not primary defense).
        </sequential_execution>
+
+       <project_root_pin>
+       {ORCHESTRATOR build-time embed: bound step-0p guard per sequential-root-pin.md — never this note}
+       </project_root_pin>
    ```
 
    The sequential mode Agent prompt uses the same structure as worktree mode but with these differences in success_criteria — since there is only one agent writing at a time, there are no shared-file conflicts:
@@ -837,8 +846,6 @@ increases monotonically across waves. `{status}` is `complete` (success),
        </success_criteria>
    ```
 
-   When worktrees are disabled for a plan (per-plan or project-level), that plan's executor runs on the main working tree. If **any** plan in the current wave dropped to sequential mode, execute the affected plan(s) **one at a time** to avoid concurrent writes to the main working tree — plans in the same wave that retained worktree isolation can still run in parallel alongside the sequential ones, but two non-worktree plans in the same wave must serialize. When the project-level `USE_WORKTREES=false`, all plans in the wave serialize regardless of the `PARALLELIZATION` setting.
-
 4. **Wait for all agents in wave to complete.**
 
    **Plan-complete heartbeat (#2410):** as each executor returns (or is verified