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>

docs/README.md

@@ -36,6 +36,7 @@ Language versions: [English](README.md) · [Português (pt-BR)](pt-BR/README.md)
 - [Isolate work with workspaces](how-to/isolate-work-with-workspaces.md) — use workspaces to sandbox experimental or risky changes
 - [Debug a failed execution](how-to/debug-a-failed-execution.md) — diagnose and recover from broken or incomplete phase execution
 - [Interpret scope-conformance warnings](how-to/interpret-scope-conformance-warnings.md) — read the advisory the worktree-wave merge emits when a plan branch commits outside its declared scope
+- [Interpret `state validate` results](how-to/interpret-state-validate-results.md) — read the `scope` reason codes and tell "nothing to report" apart from "could not look"
 - [Spike and sketch](how-to/spike-and-sketch.md) — use `/gsd-spike` and `/gsd-sketch` for exploratory work before committing to a plan
 - [Design a UI phase](how-to/design-a-ui-phase.md) — use the UI phase loop for frontend and visual work
 - [Develop a Capability for GSD 1.5+](how-to/develop-a-capability.md) — add feature Capabilities, hook fragments, and registry entries