chore(dev): document codex worktree local setup (#13494)

This PR standardizes local Codex worktree usage with a simple
dynamic-port workflow and ensures local-only artifacts stay out of
version control.

To reproduce: create a Codex worktree, run the setup script from
`.codex/environments/environment.toml`, and verify that it generates
per-worktree DB and port values along with a `Procfile.worktree` for
Overmind.

Changes included:
- Add `.codex/` and `Procfile.worktree` to `.gitignore`
- Document the Codex Worktree Workflow in `AGENTS.md`, outlining
expected local setup conventions

Tested locally by running the setup script with `CODEX_SKIP_INSTALL=1`
and `CODEX_SKIP_DB_PREPARE=1`. Verified successful output, dynamic
`FRONTEND_URL` / Vite port generation, and that `git diff` contains only
the intended documentation and ignore updates.

.gitignore

@@ -94,6 +94,7 @@ yarn-debug.log*
 .vscode
 .claude/settings.local.json
 .cursor
+.codex/
 CLAUDE.local.md
 
 # Histoire deployment
@@ -101,3 +102,4 @@ CLAUDE.local.md
 .histoire
 .pnpm-store/*
 local/
+Procfile.worktree