refactor(tests): consolidate Phase Lifecycle Module — 20 files → 4 (#3741)

* chore(tests): lint rule — cap test files per production module at 2

Adds scripts/lint-test-file-count.cjs with a ratcheted allowlist
(scripts/lint-test-file-count.allowlist.json) capturing today's
30 violating clusters as a ceiling. New entries blocked at PR time;
reductions ratchet automatically.

Wires into .github/workflows/test.yml as a new step in lint-tests.

Refs #3737

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

* refactor(tests): consolidate Phase Lifecycle Module — 20 files → 4

- Merge 10 CJS bug-fix test files into tests/phase.test.cjs
- Merge sdk/src/phase-runner-types.test.ts + sdk/src/phase-prompt.test.ts
  into sdk/src/phase-runner.test.ts (97 → 152 tests, 0 failures)
- Rename 4 mis-attributed test files out of phase cluster:
    phase-researcher-app-aware    → gsd-researcher-app-aware
    phase-researcher-flow-diagram → gsd-researcher-flow-diagram
    feat-3023-phase-type-models   → feat-3023-model-phase-types
    phase-6-cjs-sdk-seam-contracts → cjs-sdk-bridge-seam-contracts
- Fix phasePlanIndex (#3430): non-canonical plan filename warning now
  surfaces in data.warnings[] as "Ignored noncanonical plan files: ..."
  instead of a separate singular data.warning field
- Update allowlist: phase ceiling 20 → 4 (closes #3740)
- Add Phase Lifecycle Module glossary entry to CONTEXT.md

Closes #3740

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

* fix(phase-roadmap-mutation): replaceInCurrentMilestone handles active milestone inside <details> block

When the active milestone is wrapped in a <details> block (e.g. user
collapsed it, or milestone transition), the after-</details> slice is
empty or contains only footer text — the pattern never matches and the
replacement is silently dropped.

Fix: when after.replace() produces no change, fall back to replacing
inside the last <details>...</details> block. Shipped-milestone blocks
are untouched because only the last <details> block is targeted.

Closes #2641

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

* fix(changeset): add required type/pr frontmatter to 3740 fragment

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

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