Remove TestFallbackToDirectModeEnablesFlush from test skip list

The deadlock issue was already fixed in commit b8db5ab (Nov 21, 2025)
by properly initializing rootCtx in the test. The test now passes
consistently in 0.04s with no deadlocks.

Testing:
- Ran test 20 times with -count=20: all passed
- Ran with -race detector: passed
- Full test suite: all tests pass in ~35s

Closes bd-4ri

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.test-skip

@@ -1,8 +1,5 @@
 # Tests to skip due to known issues
 # Format: one test name per line (regex patterns supported)
 
-# Issue #355: Deadlocks with database mutex during cleanup
-TestFallbackToDirectModeEnablesFlush
-
 # Issue #356: Expects wrong JSONL filename (issues.jsonl vs beads.jsonl)
 TestFindJSONLPathDefault