Proposed changes

Fixes a crash where a successful run could fail at the very end with
java.nio.file.NoSuchFileException in DebugLogStore.finalizeRun() →
FileUtils.zipDir() → Files.walk().

Root cause: DebugLogStore.removeOldLogs() runs at startup over the shared
~/Library/Logs/maestro/ directory and deleted every entry beyond the 6 newest —
including the live working directory of another concurrently-running maestro
process (a separate invocation, or CLI + Studio). The owning process then tried
to zip its now-deleted directory and crashed. Long runs are most exposed, since
they drop out of the keep-window while still active.

Changes:

• Race-safe retention (DebugLogStore): replaced count-based pruning with
  pruneLogs() — keep the newest N .zip archives (completed runs) and reap
  working directories only when older than 24h (orphans from crashed runs).
  A live run's freshly-modified directory is never deleted, removing the race at
  its source. Crashed-run leftovers are still reclaimed.
• Guard FileUtils.zipDir: return early with a warning when the source
  directory does not exist, closing the check-then-walk window.
• Best-effort finalizeRun: wrap archival in try/catch that warns and
  continues — debug-log cleanup can never crash a completed run.
• Replaced a silent printStackTrace() in zipDir with a proper warning.

This is a bug fix (Type A); the one extracted helper (pruneLogs) exists to make
the retention policy unit-testable and is not a broader refactor.

Testing

Unit tests added and run via ./gradlew :maestro-client:test (all green):

• FileUtilsTest: zipDir on a missing source no longer throws and leaves no
  output file (this test reproduces the original crash as a red test); happy-path
  zip/unzip still works.
• DebugLogStoreTest: pruneLogs keeps the newest archives, deletes a stale
  orphan working dir while leaving a fresh (live) dir untouched, and no-ops on a
  missing base directory.

Lint: ./gradlew detekt passes.

Does this need e2e tests? No — this is an internal debug-logging/cleanup
fix with no change to user-facing flow behavior, so it's covered by unit tests.

Issues fixed

Fixes #1522