fix(drift): honor trailing-slash gitignore rules for absent paths

[dyk1454683243-sudo]

What

Honor directory-only gitignore rules when collecting ignored paths for MISSING_PATH.

collectIgnoredPaths asked Git via git check-ignore --stdin. Trailing-slash patterns such as local/ match directories only, and Git cannot classify a path that does not exist. On a clean checkout .mex/local was therefore not ignored — yet that is the only case MISSING_PATH can fire (when the directory is present, existsSync already skips). graph.db* already escaped because it is a glob with no trailing slash.

The ignore-collection path now also asks Git about:

• the directory form (.mex/local/)
• ignored parent prefixes (so a file under an absent ignored directory stays exempt)

The original claim is treated as ignored when any of those queries match. Real missing non-ignored paths still error.

Why

Addresses #202 (bullet 6 only).

Do not treat this as Fixes #202 or Closes #202 — that would auto-close the whole mega-issue.

Remaining #202 bullets stay open for follow-ups:

1. Symbol notation (src/auth/login.validateToken)
2. Compound extensions (.d.ts) — owned by fix(drift): treat compound extensions like .d.ts as not-paths #216
3. Hypothetical examples (./x, x.ts, x/index.ts)
4. Alternation shorthand (tests/grounding|traversal|...)
5. Scoped-package resolve (@scope/pkg/package.json vs bare specifier) — owned by fix(drift): resolve scoped packages by bare specifier #217

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. npx vitest run src/drift
2. npx vitest run test/checkers.test.ts
3. Confirm an absent .mex/local (and a path under it) is not MISSING_PATH when local/ is in .mex/.gitignore.
4. Confirm a present ignored path is still skipped, a missing non-ignored path still errors, and .mex/graph.db / graph.db* stay exempt.
5. Confirm the existing no-ignore .mex/local/ case still reports MISSING_PATH.
6. npm run typecheck

Checklist

• Focused drift tests pass (npx vitest run src/drift — 15 passed, 1 skipped #201 version-shaped case left untouched)
• Path-checker unit tests pass (npx vitest run test/checkers.test.ts — 111 passed, including absent local/ exemption, present ignored path, non-ignored missing path, and graph.db*)
• Typecheck passes (npm run typecheck)
• Tests pass (npm test) — full suite is not the gate for this ignore-collection change
• No breaking changes (or documented below)
• Tested locally with a real project

Code-graph changes

• This PR targets main
• A linked issue agrees on the bounded extractor/resolver scope
• The change follows the frozen LanguageExtractor or FrameworkResolver interface
• A focused fixture and assertions for the expected node/edge shape are included
• Any new grammar WASM, extension mapping, extractor, or resolver is registered
• No graph identity, reconciliation, schema, or drift-semantics changes are included, or a core / discuss-first issue is linked above

This is a drift path-ignore check only. It does not change graph identity, extractors, or the other #202 bullets.