Add shell completion generation (#764)

* Add shell completion generation

Add zero completions for bash, zsh, fish, PowerShell, and Elvish using one shared command tree that covers aliases, nested commands, root flags, and common exec flags.

Validate missing and unsupported shell arguments as usage errors and document safe installation examples in command help.

Refs #499

* Complete shell target suggestions

Expose every supported shell as a completions subcommand candidate and document Elvish activation.

Refs #499

* Fix Elvish completion syntax

Close generated Elvish conditional branches, use value equality, and validate generated script structure plus native Bash and Zsh syntax.

Refs #499

Author: anandh8x

internal/cli/app.go

@@ -374,6 +374,8 @@ func runWithDeps(args []string, stdout io.Writer, stderr io.Writer, deps appDeps
 	case "exec":
 		// Forward leading --add-dir occurrences so exec's own parser collects them.
 		return runExec(append(addDirFlagArgs(addDirs), args[1:]...), stdout, stderr, deps)
+	case "completions":
+		return runCompletions(args[1:], stdout, stderr)
 	case "daemon":
 		return runDaemon(args[1:], stdout, stderr, deps)
 	case "config":
@@ -1172,6 +1174,7 @@ Usage:
 
 Commands:
   exec       Run a one-shot prompt through the Go agent runtime
+  completions Generate shell completion scripts
   daemon     Manage the local background worker daemon (start/stop/status/run/attach)
   setup      Guide first-run provider setup
   config     Inspect resolved Go configuration without leaking secrets