Fix: Unknown subcommands now error instead of silently showing help

Parent commands (mol, mail, crew, polecat, etc.) previously showed help
and exited 0 for unknown subcommands like "gt mol foobar". This masked
errors in scripts and confused users.

Added requireSubcommand() helper to root.go and applied it to all parent
commands. Now unknown subcommands properly error with exit code 1.

Example before: gt mol unhook → shows help, exits 0
Example after:  gt mol unhook → "Error: unknown command "unhook"", exits 1

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/session.go

@@ -37,6 +37,7 @@ var sessionCmd = &cobra.Command{
 	Aliases: []string{"sess"},
 	GroupID: GroupAgents,
 	Short:   "Manage polecat sessions",
+	RunE:    requireSubcommand,
 	Long: `Manage tmux sessions for polecats.
 
 Sessions are tmux sessions running Claude for each polecat.