perf(adopt): scope to local branches, walk chains on demand (v0.2.25)

[rohoswagger]

Summary

ez adopt no longer scans every PR in the repo. On onyx (~10k PRs) adopting 12 local PRs went from ~120s to ~1.4s.

The old get_all_pr_statuses paginated through every PR ever opened just to find the few that matter. Now each mode does targeted fetches:

Default — ez adopt

One GraphQL call: PR statuses for local branches only. No ancestor expansion. If a local PR's base isn't local-with-PR or trunk, we warn and skip with a hint to use --pr <N> for explicit chain walking.

ez adopt feat/a feat/b

Fetch the named branches' PRs, then expand_ancestor_chains walks bases iteratively, one GraphQL batch per stack level. Bounded by stack depth.

ez adopt --pr <N>

New github::get_pr_by_number — one GraphQL call to look up the PR by number, returning head branch + PrInfo. Then expand chain like --branches.

Smoke test (onyx, ~10k PRs, 12 local branches with PRs)

ez adopt
Found 12 branch(es) to adopt
  #10429 fix-minio-default-start → … (base: `main`)
  #9083  nikg/metrics-1a-memory-delta → … (base: `main`)
  #9087  nikg/metrics-1b-redis-pool   → … (base: `nikg/metrics-1a-memory-delta`)
  #9089  nikg/metrics-1c-event-loop-lag → … (base: `nikg/metrics-1b-redis-pool`)
  ... (8 more)
✓ Adopted 12 branch(es), 0 already tracked
[ok | 1.4s]

Note the correctly-chained 3-deep stack (metrics-1a → 1b → 1c): default mode picks up multi-level stacks for free, because each level's PR base is another local branch in the fetch set.

Tests

5 new unit tests, 219 total pass:

• expand_ancestor_chains fetches missing parents until trunk (verified with a stubbed fetcher).
• Terminates when a base has no upstream PR (no infinite loop on tried set).
• No fetches when all bases are already trunk.
• orphan_local_prs flags branches with non-local non-trunk bases.
• Orphan warning order is sorted (deterministic).

Internal refactor

• expand_ancestor_chains_with(prs, trunk, fetch) takes the fetcher as a closure so tests can stub the network without setting up a fake gh.
• orphan_local_prs(prs, trunk) -> Vec<&String> extracts the "what would build_adopt_graph drop?" logic from run.
• Removed dead filter_prs_for_requested_branches and chain_to_trunk that the old global-scan flow needed.
• get_all_pr_statuses is no longer called anywhere; left in place with its existing tests for now.

Test plan

• cargo test — 219 pass
• cargo fmt --all -- --check clean
• cargo clippy -- -D warnings clean on lib code
• ez adopt smoke test on onyx — 1.4s adopts 12 local PRs including a 3-deep stack