feat(lint-org): reconcile follow-ups on write instead of appending

Every run appended a fresh dated "lint-org follow-ups" section with line-number-keyed entries, so the follow-ups file grew an unbounded pile of near-duplicate sections, kept entries whose finding had since resolved, and broke whenever the target file's line numbers shifted. Running an audit against a large todo.org surfaced exactly that drift: dead-link flags pointing at docs that now exist, and three stacked dated runs for one file.

Now lint-org rewrites the current file's section from the current run. Findings that no longer reproduce simply are not re-emitted, re-runs dedupe to one section, and entries key on checker plus message with the line as a trailing annotation, so a finding survives line shifts as the same entry. Other files' sections are left intact, and the strip step tolerates the old dated-header shape so existing follow-ups files migrate on first run. This changes the follow-ups file from an append-only log to the current outstanding findings per file.

task-audit's Phase C link-hygiene step now also reaps a matching dead-link entry when it fixes or verifies the link, scoped strictly to dead-link entries, so the audit and the follow-ups file stop drifting between lint runs.

Five follow-ups tests cover record-by-content, dedupe across runs, drop-on-resolve, and preserve-other-files. Mirrors synced.

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