enhance(#3172): require a stated failing direction for every automated acceptance command (#3825)

* test(#3172): failing-first suite for the stated failing-direction probe

Pins the <fails_when> pairing walk, placeholder denylist, MISSING sentinel
exemption, degraded-read contract, CLI arm and the plan-authoring contract text.
RED by construction: the module exports it requires do not exist yet.
Executed on the remote runner.

* feat(#3172): require a stated failing direction for every automated acceptance command

Every runnable <automated> command now carries a <fails_when> sibling naming
what output constitutes failure. A command with no expressible failure mode is
not an acceptance test: it reads as rigour and is not falsifiable.

- verify-command-grounding gains a failing-direction probe sharing the existing
  <automated> grammar, MISSING sentinel and walk guard rather than copying them
- gsd-tools check verify-failure-directions <N> backs it; plan-phase dispatches
  it and hands the JSON to gsd-plan-checker check 8f
- Dimension 8 detail extracted to references to stay under the agent size cap

Verified on the remote runner.

* fix(#3172): close four review findings in the failing-direction probe

- MISSING_SENTINEL_RE matched an env-var assignment prefix (MISSING=1 cmd), so
  a real command was exempted from the new blocking gate. Tightened the SHARED
  constant rather than adding a second copy.
- Both token regexes scanned to EOF on unclosed openers (O(n^2), 1562ms at 40k).
  Bodies are now non-crossing; 1ms, byte-identical on well-formed input. The
  pre-existing AUTOMATED_BLOCK_RE carried the same defect and is fixed here too.
- probePhaseFailingDirections reported status 'ok' when one plan was unreadable,
  conflating 'could not look' with 'nothing to report'.
- Extracted the phase-resolution block both check arms had copied verbatim.

Also corrects a docs/AGENTS.md dimension list stale since #2401.
Verified on the remote runner.

* fix(#3172): project the planner rule onto the spawn contract, settle emitted bookkeeping

The remote runner refuted the planner-side edit. agents/gsd-planner.md is frozen
under a 49152-LF-char cap asserted by four suites and sat at 49,146 — six chars
of headroom — so the +537 of authoring rule blew it. #3297/#3645 already settled
where such a rule goes: the planner spawn contract in plan-phase.md, beside
<tracked_source_paths>. The agent file is reverted to origin/next verbatim.

- plan-phase.md gains <failing_direction_contract>; tests row 30 now asserts the
  contract there and row 30b guards the freeze in both directions
- plan-phase.md growth acknowledged by APPENDING to the 3409 fragment, per the
  precedent that two ack sources may never name the same path
- install-tree fixtures regenerated for the three new reference files

Verified on the remote runner.

* chore(#3172): backfill PR number into the changeset fragment

pr:0 -> pr:3825 now that the PR exists.

---------

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

tests/fixtures/install-tree/codebuddy.json

@@ -167,6 +167,7 @@
   "gsd-core/references/execute-phase-response-language.md",
   "gsd-core/references/execute-phase-wave-guard.md",
   "gsd-core/references/executor-examples.md",
+  "gsd-core/references/failing-direction.md",
   "gsd-core/references/few-shot-examples/plan-checker.md",
   "gsd-core/references/few-shot-examples/verifier.md",
   "gsd-core/references/gate-prompts.md",
@@ -181,10 +182,12 @@
   "gsd-core/references/model-profile-resolution.md",
   "gsd-core/references/model-profiles.md",
   "gsd-core/references/mvp-concepts.md",
+  "gsd-core/references/nyquist-compliance.md",
   "gsd-core/references/offer-next.md",
   "gsd-core/references/phase-argument-parsing.md",
   "gsd-core/references/planner-antipatterns.md",
   "gsd-core/references/planner-chunked.md",
+  "gsd-core/references/planner-failing-direction.md",
   "gsd-core/references/planner-gap-closure.md",
   "gsd-core/references/planner-graphify-auto-update.md",
   "gsd-core/references/planner-guidance.md",