aitools install: document --agents auto-detection behavior

Pin down the contract for non-interactive callers (CI, agents) by
documenting that an unset --agents flag means "install for every detected
agent" outside a TTY. The selected list is already logged to stderr via
PrintInstallingFor before the install runs, so callers can verify what
was picked.

Co-authored-by: Isaac

Author: jamesbroadhead

aitools/cmd/install.go

@@ -68,6 +68,14 @@ and symlinked to each agent to avoid duplication.
 
 Use --skills name1,name2 to install specific skills.
 
+Agent selection:
+  --agents <name>[,<name>...]   Install only for the named agents.
+  (unset, interactive)          Multi-select prompt over detected agents.
+  (unset, non-interactive)      Install for every detected agent.
+
+The list of agents the command will act on is always logged to stderr before
+the install runs, so callers can verify what was picked.
+
 Supported agents: Claude Code, Cursor, Codex CLI, OpenCode, GitHub Copilot, Antigravity`,
 		Args: cobra.NoArgs,
 		RunE: func(cmd *cobra.Command, args []string) error {