Implement daemon auto-start with comprehensive improvements (bd-124)

- Auto-starts daemon on first bd command (unless --no-daemon or BEADS_AUTO_START_DAEMON=false)
- Exponential backoff on failures: 5s, 10s, 20s, 40s, 80s, 120s (max)
- Lockfile prevents race conditions when multiple commands start daemon simultaneously
- Stdio redirected to /dev/null to prevent daemon output in foreground
- Uses os.Executable() for security (prevents PATH hijacking)
- Socket readiness verified with actual connection test
- Accepts multiple falsy values: false, 0, no, off (case-insensitive)
- Working directory set to database directory for local daemon context
- Comprehensive test coverage including backoff math and concurrent starts

Fixes:
- Closes bd-1 (won't fix - compaction keeps DBs small)
- Closes bd-124 (daemon auto-start implemented)

Documentation updated in README.md and AGENTS.md

Amp-Thread-ID: https://ampcode.com/threads/T-b10fe866-ab85-417f-9c4c-5d1f044c5796
Co-authored-by: Amp <amp@ampcode.com>

README.md

@@ -518,6 +518,13 @@ bd create "Fix navbar bug"  # Uses ~/myproject/.beads/myapp.db
 bd --db ~/otherproject/.beads/other.db list
 ```
 
+### Environment Variables
+
+- `BEADS_DB` - Override database path
+- `BEADS_AUTO_START_DAEMON` - Enable/disable automatic daemon start (default: `true`). Set to `false` or `0` to disable.
+- `BD_ACTOR` - Set actor name for change tracking (defaults to `$USER`)
+- `BD_DEBUG` - Enable debug logging for troubleshooting
+
 ## Dependency Model
 
 Beads has four types of dependencies:
@@ -899,6 +906,8 @@ bd daemon --status
 bd daemon --stop
 ```
 
+**Note:** As of v0.9.11, the daemon **automatically starts** when you run any `bd` command if it's not already running. You typically don't need to manually start it. The auto-start feature can be disabled by setting `BEADS_AUTO_START_DAEMON=false` or using the `--no-daemon` flag.
+
 The daemon will:
 - Poll for changes at configurable intervals (default: 5 minutes)
 - Export pending database changes to JSONL