fix(3739): gap-checker now detects padded-prefix CONTEXT.md (#3764)

* test(3739): add RED tests for padded-prefix CONTEXT.md gap-checker miss

Covers bare and padded (01-CONTEXT.md, 02.1-CONTEXT.md) forms, an
uncovered-decision counter-test, and unit tests for the upcoming
findContextMdIn() helper. All 6 new tests fail before the fix.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(3739): extract findContextMdIn() helper; fix gap-checker bare lookup

gap-checker.cjs:136 used a bare path.join(absPhaseDir, 'CONTEXT.md')
that silently returned '' for any phase using the padded-prefix
convention (01-CONTEXT.md, 02.1-CONTEXT.md, etc.).

Extract findContextMdIn(absDir) to planning-workspace.cjs — the module
already imported by gap-checker, init, roadmap, and core — and wire
gap-checker.cjs to call it instead of the bare lookup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor(3739): replace inline dual-form predicate with findContextMdIn() at all 4 remaining sites

The dual-form predicate `f.endsWith('-CONTEXT.md') || f === 'CONTEXT.md'`
existed verbatim at 5 sites across init.cjs (×3), roadmap.cjs, and
core.cjs — Rule of Three mandates extraction at ≥3 sites. All 4
remaining call sites now delegate to findContextMdIn() from
planning-workspace.cjs. No behaviour change; all existing tests pass.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore(3739): update changeset to reference PR #3764

* fix(3739): findContextMdIn prefers bare CONTEXT.md over padded form (deterministic precedence)

`.find()` with `f.endsWith('-CONTEXT.md') || f === 'CONTEXT.md'` returned
the first match in `readdirSync` order — undefined on most filesystems.
When both `CONTEXT.md` and `01-CONTEXT.md` exist the winner was arbitrary.

The old gap-checker.cjs:136 code always used bare `CONTEXT.md` first
(existsSync on the bare path). Restore that invariant: check
`files.includes('CONTEXT.md')` before falling through to the padded scan.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test(3739): dual-file precedence test — bare CONTEXT.md wins over padded form

Adds two test cases for the scenario where both CONTEXT.md and
01-CONTEXT.md exist in the same phase directory:

1. Helper level: findContextMdIn() must return 'CONTEXT.md' (not the
   padded filename) when both files are present on disk.
2. Integration level: gap-analysis must resolve decisions from the bare
   form only; D-PADDED (from 01-CONTEXT.md) must not appear when the
   bare form shadows it.

Without these tests a future change to findContextMdIn could silently
regress the precedence guarantee.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(3739): bug-2798 tests skip cleanly when sdk/dist is absent (was hard-fail)

The 3 tests in bug-2798-context-window-config-key.test.cjs invoke the built
SDK CLI (sdk/dist/cli.js) and require sdk/dist/query/config-schema.js. When
dist is absent, they threw hard errors rather than observable skips.

Apply the same `if (!existsSync(...)) { t.skip(...); return; }` guard used in
bug-2767-gsd-sdk-commit-files-flag.test.cjs (c2812313). Tests 1 & 2 guard on
sdk/dist/cli.js; test 3 guards on sdk/dist/query/config-schema.js. When dist
is present all 3 run; when absent all 3 emit actionable skip lines.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(3739): eliminate double readdirSync in findContextMdIn callers

findContextMdIn now accepts either a directory path or an already-read
files array, allowing callers that already hold a directory listing
(core.cjs:getPhaseFileStats, roadmap.cjs:countPhasePlansAndSummaries,
gap-checker.cjs:runGapAnalysis) to skip redundant readdirSync calls.

Test coverage added for the array-argument overload.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: redesign flaky concurrent add-blocker test with deterministic barrier

Previous design relied on OS scheduler to interleave two subprocess writes,
producing a flake under CI load. Redesigned using a file-barrier (Option A)
that forces both subprocesses to reach a ready-gate before either proceeds,
guaranteeing true concurrent lock contention and eliminating timing dependency.

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

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>