gaps: default-ignore patterns for manifest/generated files + path-parsing artifact

[Wynelson94]

Problem

fieldnotes gaps ranks every churning uncovered file, which puts package-lock.json, CHANGELOG.md, README.md, version manifests, etc. at the top of real repos — files whose churn says nothing about missing knowledge. First run on a production Next.js repo: 3 of the top 5 "gaps" were manifest noise.

Proposal

• Built-in default ignores (lockfiles, CHANGELOG*, LICENSE*, .github/, generated-file globs), plus a gaps.ignore list in .fieldnotes/config.toml for per-repo additions.
• --no-ignore to see everything.
• brief's ambient coverage-gap line should use the same filter (it currently can surface a lockfile as the hottest gap).

Related parsing artifact

On one repo, gaps printed a path with a doubled slash (src/app/api/tasks//route.ts) — likely a rename entry in git log --name-only output that churn_map (fieldnotes/gaps.py) doesn't normalize. Normalize paths (and handle old => new rename lines) while in there.

[Wynelson94]

Re: the "Related parsing artifact" section — the doubled slash isn't a churn_map / git log --name-only parsing bug. It's Rich markup eating the bracketed segment at print time:

$ python3 -c "from rich.console import Console; Console().print('src/app/api/prospects/[id]/route.ts')"
src/app/api/prospects//route.ts

That's why it also shows up in get, stale, and verify, which never touch git log. Filed as #6 with the fix (markup=False, per the existing cli.py:800 pattern).

The default-ignore proposal in this issue stands on its own — confirmed again on a Next.js repo today, where gaps ranked package.json (23 commits), package-lock.json (22), and CLAUDE.md (20) as the top three uncovered "gaps".