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>

cmd/bd/daemon_autostart.go

@@ -103,7 +103,7 @@ func restartDaemonForVersionMismatch() bool {
 		return false
 	}
 
-	args := []string{"daemon"}
+	args := []string{"daemon", "--start"}
 	cmd := exec.Command(exe, args...)
 	cmd.Env = append(os.Environ(), "BD_DAEMON_FOREGROUND=1")
 
@@ -267,7 +267,7 @@ func startDaemonProcess(socketPath string, isGlobal bool) bool {
 		binPath = os.Args[0]
 	}
 
-	args := []string{"daemon"}
+	args := []string{"daemon", "--start"}
 	if isGlobal {
 		args = append(args, "--global")
 	}