feat(cli): add agent start to launch named agents in managed tmux sessions

[codeaholicguy]

Summary

• Add npx ai-devkit agent start --type <type> [--name <name>] [--cwd <path>] to launch claude / codex / gemini_cli / opencode in a detached tmux session and register it in a local file (~/.ai-devkit/agents.json).
• agent list, agent send, and agent open now resolve the user-supplied name (registry-first), instead of showing the opaque {folder}-{pid} auto-name.

Test plan

• Manual smoke: agent start --type claude --name foo on macOS, verify ~/.ai-devkit/agents.json records the real claude PID (not the wrapper) and agent list shows foo.
• Manual smoke: same flow with --type gemini_cli to confirm the Node-script matcher resolves to the gemini process.
• Manual smoke: kill the agent inside the pane, rerun agent start with the same name, confirm warning + replacement path.