refactor(tests): consolidate Installer Module — 11 files → 2 (#3760)

* 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>

* chore(tests): add docs-exempt to changeset fragment

Internal CI lint rule — no user-facing docs impact.

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

* refactor(tests): consolidate Installer Module — 11 files → 2

Closes #3758

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

* fix(changeset): replace invalid type 'Chore' with 'Changed' — ALLOWED_TYPES only accepts Added|Changed|Deprecated|Removed|Fixed|Security

Fragment already had a docs-exempt marker; only the type value was wrong.

Refs #3758

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

* refactor(tests): split install.test.cjs along section seams — keep files ≤ 800 LOC

1828-LOC monolith split into 3 files by semantic boundary:
  install.test.cjs (602 LOC) — S1-5: dir resolution, install/uninstall spot-checks, Kilo
  install-runtime-artifacts.test.cjs (347 LOC) — S6-8+12: layout loop, Contract 6, legacy migrations
  install-minimal-hooks.test.cjs (782 LOC) — S9-11+13: profiles, minimal E2E, hooks copy

Shared constants and helpers extracted to tests/helpers/install-shared.cjs (216 LOC).
Total test count unchanged: 210 tests (68+37+105).

Refs #3758

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

* fix(tests): ratchet state test-file ceiling to 9 — actual count at baseline

The allowlist was initialised with state=8 but the repo already had 9 files
matching the 'state' prefix at the time the lint rule was created, causing
lint-test-file-count.test.cjs to fail on the real repo immediately.

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

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