refactor(#865): extract pure phase-id helpers from core.cts into phase-id.cts (#868)

ADR-857 rollout phase 2a — the first cut of the core.cts decomposition.
Move the 9 pure phase-id parsing/matching helpers (escapeRegex,
normalizePhaseName, comparePhaseNum, extractPhaseToken, phaseTokenMatches,
phaseMarkdownRegexSource/Exact, getMilestoneFromPhaseId, getPhaseDirFromPhaseId)
out of core.cts into a new leaf module src/phase-id.cts. core.cts re-exports
them (behavior-preserving); its internal callers resolve the destructured
bindings.

Cycle-safe by design: phase-id depends on nothing in core, so re-export creates
no circular require (the property that made phase-1 io.cts clean). This is the
leaf-first ordering — it unblocks the roadmap-parser extraction (2b), which
imports phaseMarkdownRegexSource.

New-CLI-module checklist: .gitignore, eslint.config.mjs ignores, INVENTORY.md
count 91->92 + row, INVENTORY-MANIFEST.json, ARCHITECTURE.md row, CONTEXT.md
"Phase Id Module" glossary entry. Adds tests/phase-id.test.cjs (63 behavioral
tests incl. shim-identity + adversarial inputs).

Gates: lint, code-review, security-review, codex adversarial-review (all 0
findings), and gsd-test-both (14814 pass on Mac + Linux Docker, 0 fail).

Closes #865

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

eslint.config.mjs

@@ -90,6 +90,7 @@ export default tseslint.config(
       'gsd-core/bin/lib/command-routing-hub.cjs',
       'gsd-core/bin/lib/core.cjs',
       'gsd-core/bin/lib/io.cjs',
+      'gsd-core/bin/lib/phase-id.cjs',
       'gsd-core/bin/lib/drift.cjs',
       'gsd-core/bin/lib/cjs-command-router-adapter.cjs',
       'gsd-core/bin/lib/phase-command-router.cjs',