perf: cache doctor beads dir resolution

[osamu2001]

Summary

• follow up on @DreadPirateRobertz review feedback from fix: support bare-parent worktree beads fallback #2617
• simplify the doctor resolver flow by removing the extra indirection
• cache beads dir resolution so the fallback git lookup runs once per doctor invocation

Testing

• go test ./cmd/bd/doctor -run 'Test(ResolveBeadsDirForRepo_BareParentWorktreeFallback|ResolveBeadsDirForRepo_CachesFallbackResult|CheckMetadataVersionTracking_BareParentWorktreeFallback|CheckLockHealth_BareParentWorktreeFallback|CheckDoltLocks_BareParentWorktreeFallback)$'
• go run ./cmd/bd doctor with a git wrapper confirmed fallback rev-parse calls dropped from 28 to 1

Context

This is a follow-up to the review on #2617 after that branch had effectively already landed in main.

[codecov-commenter]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
4806	1	4805	548

View the top 1 failed test(s) by shortest run time

github.com/steveyegge/beads/cmd/bd::TestGetVersionsSinceOrder

Stack Traces | 0s run time

=== RUN   TestGetVersionsSinceOrder
    version_tracking_test.go:80: Versions not in chronological order: 0.61.0 (2026-03-15) should come before 0.57.0 (2026-03-01)
--- FAIL: TestGetVersionsSinceOrder (0.00s)

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