feat(extensions/pi): ship pi-env.zsh shell loader

The loader that sources ~/.config/pi/.env into every shell was only
living in the myconfigs tor-ms22 backup \u2014 a fresh machine had nowhere
to get it from except copying by hand. Now canonical here.

- extensions/pi/pi-env.zsh: 20-line POSIX-compatible loader
  (set -a; source ~/.config/pi/.env; set +a). Works in bash and zsh.
- install.sh install_pi_env_loader:
  * oh-my-zsh detected (~/.oh-my-zsh/custom/ exists)
    \u2192 cp into that dir (NOT symlink \u2014 that dir is typically part of
      a dotfiles backup, and a symlink to mempalace-toolkit would
      break when restored on another host).
    \u2192 Idempotent: if target content matches repo, says 'already
      installed'. If it differs, leaves user edits alone and points
      at diff for manual reconcile.
  * No oh-my-zsh \u2192 prints source-this-line snippet for ~/.zshrc or
    ~/.bashrc (derived from $SHELL). Does NOT auto-edit rc files.
- install.sh uninstall: only removes the copy if content still matches
  repo. Local edits preserved.
- Docs:
  * extensions/pi/README.md Environment setup section rewritten with
    both install paths, step 5 of deploy recipe updated.
  * AGENTS.md Structure block lists pi-env.zsh.
  * Root README repo-contents line mentions it.

Verified on tor-ms22: install fresh \u2192 uninstall (content match \u2192 remove)
\u2192 reinstall \u2192 zsh -ic loads AWS vars correctly. Also tested bash fallback
path via HOME=/tmp/fake-home SHELL=/bin/bash \u2014 prints right .bashrc snippet.

AGENTS.md

@@ -22,6 +22,7 @@ extensions/
   pi/                   # pi coding-agent bring-up: MCP bridge, keybindings, settings template
     mempalace.ts        # Symlinked into ~/.pi/agent/extensions/ (MCP <→ pi glue)
     keybindings.json    # Symlinked into ~/.pi/agent/ (mosh/tmux newline fix)
+    pi-env.zsh          # Copied into ~/.oh-my-zsh/custom/ or sourced from .bashrc (loads ~/.config/pi/.env)
     settings.example.json # Template; user copies + edits (pi rewrites settings.json at runtime)
     README.md           # Extension internals, schema-passthrough gotcha, env setup
 ```