feat(extensions/pi): keybindings symlink + settings template + AWS/pi probes

Round out the pi bring-up story so a fresh machine can reach a working
pi+mempalace install with just `git clone && ./install.sh`:

- extensions/pi/keybindings.json: generic mosh/tmux newline fix
  (shift+enter, ctrl+j, alt+j). Safe on any machine — not
  region/account-specific. Symlinked into ~/.pi/agent/.
- extensions/pi/settings.example.json: template for `settings.json`
  so pi can start without --provider/--model. NOT symlinked — pi
  rewrites settings.json at runtime (lastChangelogVersion bumps),
  which would dirty the repo. Installer prints the cp + edit hint.
- install.sh: new install_pi_keybindings + uninstall mirror; new
  check_pi_settings probe (warns if settings.json missing); new
  check_aws_env probe (warns if AWS_PROFILE/AWS_REGION unset and
  settings.json selects amazon-bedrock). All new steps gated on
  pi being installed (~/.pi/agent/extensions/ exists).
- extensions/pi/README.md: documents keybindings rationale,
  settings bootstrap, and the recommended ~/.config/pi/.env +
  ~/.oh-my-zsh/custom/pi-env.zsh env layout (paired with the
  myconfigs commit 884e329 that split AWS vars out of
  ~/.config/opencode/.env).

Verified on tor-ms22: full install → uninstall → reinstall cycle,
new shell loads AWS_PROFILE/AWS_REGION from the new pi-env.zsh hook.

Works on macOS and Linux (plain ln -s, POSIX bash).

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/) — the [pi coding-agent](https://github.com/mariozechner/pi-coding-agent) extension that bridges pi to the MemPalace MCP server (wake-up auto-injection, `/mempalace-diary` command, schema passthrough). Symlinked into `~/.pi/agent/extensions/` on install when pi is detected.
+- [`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.
 
 **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).**