feat(api): link adoption ledger for a deterministic noise guard (#709)

[Zach Dunn (zachdunn)]

Follow-up to #701/#707. Adds a per-target D1 ledger (github_adopted_links,
mirroring github-ingest-ledger.ts's pattern) so the link-adoption noise
guard is deterministic across passes and supports un-adoption, closing #709.

What changed

• New ledger (apps/api/src/github-link-adopt-ledger.ts,
  apps/api/migrations/20260818200000_github_adopted_links.sql): one row per
  (repo, kind, num, source_key) — the resolved workspace key a pasted
  uploads.sh link pointed to, scoped to the PR/issue it was adopted into.
  detached_at NULL means currently referenced by source (the body or the
  specific comment it was last seen in).
• Noise guard now counts from the ledger: shouldSyncAfterAdopt reads the
  TOTAL non-detached rows for the target, not the current pass's copy count,
  so two links pasted in two separate comments deterministically trip the
  ≥2 threshold on the second one.
• Idempotent, cheaply: a rescan (redelivery, comment edit) that finds a
  key already ledgered and not detached skips re-attaching entirely — no
  re-copy, no metadata rewrite.
• Un-adoption parity with ingest's detach: adoptLinkedFiles now
  reconciles per-source (body or one comment) — a ledgered key no longer
  referenced when that exact source is rescanned gets marked detached and
  the managed comment resyncs. The copy is never deleted, only hidden from
  the render via a gh.detached metadata flag; gatherAttachments
  (github-comment.ts) now filters on it unconditionally. A re-pasted link
  un-detaches without a re-copy.

Pinning test

Before touching the guard, I wrote a test pinning the issue's own scenario
(two links pasted in two separate comments, one at a time) against the
pre-ledger code. It already synced correctly on the second comment — the
first comment's copy landed in R2 regardless of whether the guard fired,
so gatherCommentBody's pre-adoption listing saw it as a "pre-existing
attachment" and tripped the guard by coincidence. Kept as regression
coverage (github-link-adopt.test.ts, "PIN (issue #709)") now that the
guard reads the ledger directly instead of relying on that side effect.

Tests

apps/api/src/github-link-adopt.test.ts — ledger-backed idempotency, the
pinning test, and the detach/re-adopt round-trip.
apps/api/test/github-link-adopt-ledger-sqlite.test.ts — real-SQL-semantics
coverage for the ledger, mirroring github-ingest-ledger-sqlite.test.ts.
apps/api/src/github-comment.test.ts — updated for the unconditional
gh.detached filter read.

pnpm test (4536 tests) and pnpm -r typecheck both green.

No changeset — this is server-only (apps/api), no published-package
(@buildinternet/uploads) behavior changed.

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