feat: Add --start flag to bd daemon, show help with no args

Currently 'bd daemon' with no args immediately starts the daemon. This is
inconsistent with other daemon management commands like --stop, --status,
etc. and makes the command less discoverable for new users.

Changes:
- Add --start flag to explicitly start daemon
- Show help text when no operation flags provided
- Update auto-start logic to use --start flag
- Update startDaemon() to pass --start when forking
- Update all documentation to use 'bd daemon --start'
- Update MCP Python client error messages

The MCP docs already incorrectly showed 'bd daemon start' which doesn't
work, so this change fixes that documentation bug while improving UX.

Auto-start still works correctly - it now passes --start internally.

Fixes bd-gfu

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

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

docs/DAEMON.md

@@ -173,7 +173,7 @@ FileWatcher (platform-native)
 
 ```bash
 # Enable for single daemon
-BEADS_DAEMON_MODE=events bd daemon start
+BEADS_DAEMON_MODE=events bd daemon --start
 
 # Set globally in shell profile
 export BEADS_DAEMON_MODE=events
@@ -200,14 +200,14 @@ bd daemons killall
 
 ```bash
 # Fail if watcher unavailable (e.g., testing)
-BEADS_WATCHER_FALLBACK=false BEADS_DAEMON_MODE=events bd daemon start
+BEADS_WATCHER_FALLBACK=false BEADS_DAEMON_MODE=events bd daemon --start
 ```
 
 **Switch back to polling:**
 
 ```bash
 # Explicitly use polling mode
-BEADS_DAEMON_MODE=poll bd daemon start
+BEADS_DAEMON_MODE=poll bd daemon --start
 
 # Or unset to use default
 unset BEADS_DAEMON_MODE
@@ -280,7 +280,7 @@ bd info --json | grep daemon_running
 export BEADS_AUTO_START_DAEMON=false
 
 # Start manually
-bd daemon start
+bd daemon --start
 ```
 
 **Auto-start with exponential backoff:**