fix: tolerate invalid sync candidates

[PANGKAIFENG]

Summary

• isolate invalid model candidates instead of failing the full sync scan
• preserve deterministic fallback for explicit #待办 records
• retry a transient Claude timeout once while preserving hard failures

Verification

• pnpm test (56 files, 579 tests)
• pnpm typecheck
• pnpm lint
• pnpm build:obsidian
• pnpm build:runner
• read-only full ClawVault scan: 34 files, 76 records, 34 candidates, exit 0
• Inbox/Active counts unchanged and capture checkpoint unchanged after read-only scan
• reload Obsidian and inspect the candidate modal (blocked only by macOS lock screen)

Closes #59