fix(state): make SQLite side-file hardening race-safe

[munezaclovis]

What changed

• Harden the persistent pv.db file unconditionally.
• Treat NotFound as benign only when SQLite removes pv.db-wal or pv.db-shm during hardening.
• Preserve typed failures for a missing persistent database, unsafe permissions, wrong ownership, and other filesystem errors.
• Add deterministic state coverage for side-file disappearance and daemon integration coverage for watcher liveness and job state.

Why

SQLite may remove its WAL and shared-memory files between discovery and the permission/ownership checks. The resulting NotFound escaped through the project config watcher and could stop the daemon.

The persistent database is not ephemeral, so its disappearance remains an error. No retries or ownership checks were weakened.

Review commits

1. fix(state): require persistent database hardening
2. fix(state): tolerate disappearing SQLite side files
3. test(daemon): cover disappearing SQLite side files

Checks

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features --locked -- -D warnings
• cargo nextest run -p state (87 passed)
• cargo test -p daemon --test daemon_foundation (31 passed)

Closes #309

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[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing agent/fix-sqlite-aux-hardening-race (05230cd) with main (058373c)}