refactor(#3187): give state field extraction one canonical owner (#3283)

* refactor(#3187): give state field extraction one canonical owner

stateFieldValue in state-document.cts becomes the single owner of the #1760
frontmatter-then-body fallback chain. The new whole-repo guard found 14
independent re-derivations where the epic scoped 5, all now routed through it:
cmdStateSnapshot (11), cmdStatePrune (2) and smart-entry fmScalar (1).

state validate was a gate that could not fail. Every warning it could emit sat
behind a phase resolved without the frontmatter tier, so a STATE.md whose phase
lives only in frontmatter skipped the drift scan entirely and returned
valid:true. It also read unstripped content, letting a frontmatter status: key
shadow the body field (#1255 class). Both fixed; output gains a scope field so
could-not-look stops being output-identical to looked-and-clean.

Verified on the remote runner.

* docs(#3187): document the state validate scope field and its reason codes

Adds docs/how-to/interpret-state-validate-results.md so a reader can tell
nothing-to-report from could-not-look, updates the COMMANDS.md and USER-GUIDE.md
entries, corrects the CONTEXT.md glossary overstatement about Current Position
sole ownership, and drops the changeset fragment.

* fix(#3187): close three drift-guard evasion shapes and test the refuse path

The isolated adversarial review found the ladder detector was evadable by
ordinary reformatting, not just deliberately: a member or computed operand
(fm.key / fm[key]) missed the bare-identifier backreference, a swapped tier
order missed a hardcoded number-then-boolean sequence, and a ladder wrapped
across lines missed single-line detection. All three now caught, each with its
own test plus a proven boundary control.

The frontmatter-parse refuse path on the destructive complete-phase route was
unreachable and therefore untested. It is now driven by an injected parse
failure and asserts STATE.md is byte-identical after the refusal, rather than
shipping untested defensive code on a path that rewrites user state.

Verified on the remote runner.

* fix(#3187): widen the drift guard to the prompt layer and disclose tier-2 changes

The code-review spec axis found the guard's scan surface was src/ only, which is
Decision 4(d)'s forbidden allowlist one directory wide - and it had a live miss:
gsd-core/workflows/smart-entry.md tells an agent to read status from frontmatter
or the body, a prose expression of this same chain. The surface now covers the
prompt layer. That one site carries a permanent written exemption rather than a
ratchet: it is the gsd-tools-is-down fallback, so it cannot call the owner by
construction, and a ratchet would imply removable debt that does not exist.

Two tier-2 output changes shipped undisclosed and are now named in the changeset
and docs: complete-phase's idempotency guard consulting frontmatter, and the
workstream inventory resolving frontmatter-only fields. docs/COMMANDS.md gains a
state complete-phase entry, which it never had.

Also records Amendment 5 on ADR-3180, extracts the duplicated frontmatter-parse
block the epic's own thesis forbids, and re-points two assertions from free-form
warning prose onto the structured drift object.

Verified on the remote runner.

* chore(#3187): backfill changeset PR number

pr:0 placeholder replaced with the real PR number now that #3283 exists.

---------

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

package.json

@@ -112,7 +112,7 @@
     "lint": "eslint . --cache --cache-location node_modules/.cache/eslint/",
     "lint:fix": "eslint . --fix",
     "lint:table-schema-drift": "node scripts/lint-table-schema-drift.cjs",
-    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-emitted-drift-ack.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs && node scripts/lint-fix-has-regression-test.cjs && node scripts/lint-example-parser-parity.cjs && node scripts/lint-docs-command-form.cjs && node scripts/lint-plan-count-drift.cjs && node scripts/lint-milestone-window-drift.cjs && node scripts/lint-phase-enumeration-drift.cjs && node scripts/lint-planning-prompt-drift.cjs && node scripts/lint-completion-ratio-drift.cjs",
+    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-emitted-drift-ack.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs && node scripts/lint-fix-has-regression-test.cjs && node scripts/lint-example-parser-parity.cjs && node scripts/lint-docs-command-form.cjs && node scripts/lint-plan-count-drift.cjs && node scripts/lint-milestone-window-drift.cjs && node scripts/lint-phase-enumeration-drift.cjs && node scripts/lint-planning-prompt-drift.cjs && node scripts/lint-completion-ratio-drift.cjs && node scripts/lint-state-field-drift.cjs",
     "lint:allow-test-rule-refs": "node scripts/lint-allow-test-rule-refs.cjs",
     "lint:regression-names": "node scripts/lint-regression-test-names.cjs",
     "lint:descriptions": "node scripts/lint-descriptions.cjs",