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.

README.md

@@ -8,7 +8,7 @@ Producer-side tooling for [MemPalace](https://github.com/MemPalace/mempalace)
 - `bin/mempalace-docs` — mines project directories into MemPalace while excluding source code, keeping the palace signal-dense.
 - [`ARCHITECTURE.md`](ARCHITECTURE.md) — **canonical spec**: architecture diagram, component details, setup recipe, operational notes, upstream-retirement roadmap.
 - [`SKILL.md`](SKILL.md) — the companion agent skill, symlinked into `~/.agents/skills/opencode-mempalace-bridge/` on install.
-- [`extensions/pi/`](extensions/pi/) — pi coding-agent bridge: the MemPalace MCP extension (symlinked), a mosh/tmux-friendly keybindings file (symlinked), and a `settings.example.json` template for starting pi without `--model`. `install.sh` also probes for `AWS_PROFILE`/`AWS_REGION` (needed by pi's Bedrock provider) and points at the recommended `~/.config/pi/.env` layout if missing.
+- [`extensions/pi/`](extensions/pi/) — pi coding-agent bridge: the MemPalace MCP extension (symlinked), a mosh/tmux-friendly keybindings file (symlinked), a `pi-env.zsh` shell loader (copied into `~/.oh-my-zsh/custom/` on zsh+omz hosts, or source'd from `.bashrc` / `.zshrc` otherwise), and a `settings.example.json` template for starting pi without `--model`. `install.sh` also probes for `AWS_PROFILE`/`AWS_REGION` (needed by pi's Bedrock provider) and points at the recommended `~/.config/pi/.env` layout if missing.
 
 **If you're just trying to get this working on a new machine → jump to [Setup](#setup).**
 **If you want the full architecture story → read [`ARCHITECTURE.md`](ARCHITECTURE.md).**