feat: auto-remap codebase after significant phase execution (closes #2003) (#2605)

* feat: auto-remap codebase after significant phase execution (#2003)

Adds a post-phase structural drift detector that compares the committed tree
against `.planning/codebase/STRUCTURE.md` and either warns or auto-remaps
the affected subtrees when drift exceeds a configurable threshold.

## Summary
- New `bin/lib/drift.cjs` — pure detector covering four drift categories:
  new directories outside mapped paths, new barrel exports at
  `(packages|apps)/*/src/index.*`, new migration files, and new route
  modules. Prioritizes the most-specific category per file.
- New `verify codebase-drift` CLI subcommand + SDK handler, registered as
  `gsd-sdk query verify.codebase-drift`.
- New `codebase_drift_gate` step in `execute-phase` between
  `schema_drift_gate` and `verify_phase_goal`. Non-blocking by contract —
  any error logs and the phase continues.
- Two new config keys: `workflow.drift_threshold` (int, default 3) and
  `workflow.drift_action` (`warn` | `auto-remap`, default `warn`), with
  enum/integer validation in `config-set`.
- `gsd-codebase-mapper` learns an optional `--paths <p1,p2,...>` scope hint
  for incremental remapping; agent/workflow docs updated.
- `last_mapped_commit` lives in YAML frontmatter on each
  `.planning/codebase/*.md` file; `readMappedCommit`/`writeMappedCommit`
  round-trip helpers ship in `drift.cjs`.

## Tests
- 55 new tests in `tests/drift-detection.test.cjs` covering:
  classification, threshold gating at 2/3/4 elements, warn vs. auto-remap
  routing, affected-path scoping, `--paths` sanitization (traversal,
  absolute, shell metacharacter rejection), frontmatter round-trip,
  defensive paths (missing STRUCTURE.md, malformed input, non-git repos),
  CLI JSON output, and documentation parity.
- Full suite: 5044 pass / 0 fail.

## Documentation
- `docs/CONFIGURATION.md` — rows for both new keys.
- `docs/ARCHITECTURE.md` — section on the post-execute drift gate.
- `docs/AGENTS.md` — `--paths` flag on `gsd-codebase-mapper`.
- `docs/USER-GUIDE.md` — user-facing behavior note + toggle commands.
- `docs/FEATURES.md` — new 27a section with REQ-DRIFT-01..06.
- `docs/INVENTORY.md` + `docs/INVENTORY-MANIFEST.json` — drift.cjs listed.
- `get-shit-done/workflows/execute-phase.md` — `codebase_drift_gate` step.
- `get-shit-done/workflows/map-codebase.md` — `parse_paths_flag` step.
- `agents/gsd-codebase-mapper.md` — `--paths` directive under parse_focus.

## Design decisions
- **Frontmatter over sidecar JSON** for `last_mapped_commit`: keeps the
  baseline attached to the file, survives git moves, survives per-doc
  regeneration, no extra file lifecycle.
- **Substring match against STRUCTURE.md** for `isPathMapped`: the map is
  free-form markdown, not a structured manifest; any mention of a path
  prefix counts as "mapped territory". Cheap, no parser, zero false
  negatives on reasonable maps.
- **Category priority migration > route > barrel > new_dir** so a file
  matching multiple rules counts exactly once at the most specific level.
- **Empty-tree SHA fallback** (`4b825dc6…`) when `last_mapped_commit` is
  absent — semantically correct (no baseline means everything is drift)
  and deterministic across repos.
- **Four layers of non-blocking** — detector try/catch, CLI try/catch, SDK
  handler try/catch, and workflow `|| echo` shell fallback. Any single
  layer failing still returns a valid skipped result.
- **SDK handler delegates to `gsd-tools.cjs`** rather than re-porting the
  detector to TypeScript, keeping drift logic in one canonical place.

Closes #2003

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

* docs(mapper): tag --paths fenced block as text (CodeRabbit MD040)

Comment 3127255172.

* docs(config): use /gsd- dash command syntax in drift_action row (CodeRabbit)

Comment 3127255180. Matches the convention used by every other command
reference in docs/CONFIGURATION.md.

* fix(execute-phase): initialize AGENT_SKILLS_MAPPER + tag fenced blocks

Two CodeRabbit findings on the auto-remap branch of the drift gate:

- 3127255186 (must-fix): the mapper Task prompt referenced
  ${AGENT_SKILLS_MAPPER} but only AGENT_SKILLS (for gsd-executor) is
  loaded at init_context (line 72). Without this fix the literal
  placeholder string would leak into the spawned mapper's prompt.
  Add an explicit gsd-sdk query agent-skills gsd-codebase-mapper step
  right before the Task spawn.
- 3127255183: tag the warn-message and Task() fenced code blocks as
  text to satisfy markdownlint MD040.

* docs(map-codebase): wire PATH_SCOPE_HINT through every mapper prompt

CodeRabbit (review id 4158286952, comment 3127255190) flagged that the
parse_paths_flag step defined incremental-remap semantics but did not
inject a normalized variable into the spawn_agents and sequential_mapping
mapper prompts, so incremental remap could silently regress to a
whole-repo scan.

- Define SCOPED_PATHS / PATH_SCOPE_HINT in parse_paths_flag.
- Inject ${PATH_SCOPE_HINT} into all four spawn_agents Task prompts.
- Document the same scope contract for sequential_mapping mode.

* fix(drift): writeMappedCommit tolerates missing target file

CodeRabbit (review id 4158286952, drift.cjs:349-355 nitpick) noted that
readMappedCommit returns null on ENOENT but writeMappedCommit threw — an
asymmetry that breaks first-time stamping of a freshly produced doc that
the caller has not yet written.

- Catch ENOENT on the read; treat absent file as empty content.
- Add a regression test that calls writeMappedCommit on a non-existent
  path and asserts the file is created with correct frontmatter.
  Test was authored to fail before the fix (ENOENT) and passes after.

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/ARCHITECTURE.md

@@ -467,8 +467,8 @@ Equivalent paths for other runtimes:
 │   ├── ARCHITECTURE.md
 │   └── PITFALLS.md
 ├── codebase/               # Brownfield mapping (from /gsd-map-codebase)
-│   ├── STACK.md
-│   ├── ARCHITECTURE.md
+│   ├── STACK.md            # YAML frontmatter carries `last_mapped_commit`
+│   ├── ARCHITECTURE.md     # for the post-execute drift gate (#2003)
 │   ├── CONVENTIONS.md
 │   ├── CONCERNS.md
 │   ├── STRUCTURE.md
@@ -502,6 +502,30 @@ Equivalent paths for other runtimes:
 └── continue-here.md        # Context handoff (from pause-work)
 ```
 
+### Post-Execute Codebase Drift Gate (#2003)
+
+After the last wave of `/gsd:execute-phase` commits, the workflow runs a
+non-blocking `codebase_drift_gate` step (between `schema_drift_gate` and
+`verify_phase_goal`). It compares the diff `last_mapped_commit..HEAD`
+against `.planning/codebase/STRUCTURE.md` and counts four kinds of
+structural elements:
+
+1. New directories outside mapped paths
+2. New barrel exports at `(packages|apps)/<name>/src/index.*`
+3. New migration files
+4. New route modules under `routes/` or `api/`
+
+If the count meets `workflow.drift_threshold` (default 3), the gate either
+**warns** (default) with the suggested `/gsd:map-codebase --paths …` command,
+or **auto-remaps** (`workflow.drift_action = auto-remap`) by spawning
+`gsd-codebase-mapper` scoped to the affected paths. Any error in detection
+or remap is logged and the phase continues — drift detection cannot fail
+verification.
+
+`last_mapped_commit` lives in YAML frontmatter at the top of each
+`.planning/codebase/*.md` file; `bin/lib/drift.cjs` provides
+`readMappedCommit` and `writeMappedCommit` round-trip helpers.
+
 ---
 
 ## Installer Architecture