feat(pi): add Pi (pi.dev) as a sixth CLI run mode (#206)

SessionMode gains 'pi', a first-class backend alongside Claude Code,
OpenCode, Codex, Gemini and Antigravity: its own PTY, tmux session, rose
tab identity, welcome button, run-mode entry, cron agentType, Docker and
remote-SSH command defaults, and clone-repo Brain option.

Pi is a different shape of CLI from the other four, and three decisions
follow from that:

- It has NO permission prompts and no sandbox, so there is no
  --dangerously-skip-permissions analog and none was invented. The
  privilege-shaped knob is the tri-state approveProjectTrust, which makes
  pi load and EXECUTE repo-local .pi/extensions TypeScript and install
  missing project packages. clampExternalCliBypassForOwner() therefore
  puts pi in the MATERIALIZE branch: a non-granted multi-user owner gets
  --no-approve even when no config was sent, because pi's own default is
  a prompt the session user could answer themselves. That helper had zero
  test coverage; it now has coverage for all four CLIs.
- Only the PI_ prefix joins the env allowlist. Pi's ~34 provider key vars
  share no prefix and ALLOWED_ENV_PREFIXES is one global list with no mode
  context, so admitting them would widen the allowlist for every mode at
  once. Auth goes through pi's /login or the server's own environment.
  --api-key is deliberately never wired: it would put a provider secret on
  the spawn command line.
- pi stays OUT of isAltScreenStripMode(). Its default TUI renders into the
  main screen with terminal-owned scrollback, and its 0.84.0 fullscreen
  mode is runtime-switchable via /settings; that flip was measured to put
  the pane into the alt screen, which the strip would have corrupted.

pi-cli-resolver.ts additionally sanity-probes `pi --version` and requires
semver-shaped output, because `pi` is a short generic name a stray binary
can shadow; GET /api/pi/status surfaces path and version so a
misresolution is diagnosable rather than presenting as a broken mode.

Docker installs pi in its own --ignore-scripts step so that flag cannot
affect the other four CLIs, and seeds its credentials per-file rather than
whole-dir (~/.pi/agent also holds sessions, extensions and package trees).

Verified end to end against pi 0.84.1 on an isolated instance: resolver
search-dir fallback, flag construction, piConfig persistence across a full
server restart, the trust prompt and its --no-approve suppression, the
rose Run button on the default daylight-blue skin (the nested skin block
eats per-mode gradients unless the rule lives inside it), and the buffer
local-echo policy, which pi tolerates where codex did not.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.changeset/ba4bc996.md

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+---
+"aicodeman": minor
+---
+
+Add Pi (pi.dev) as a sixth CLI run mode (#206).
+
+`SessionMode` gains `'pi'`, a first-class backend alongside Claude Code, OpenCode, Codex, Gemini and Antigravity: its own PTY, tmux session, rose tab identity, welcome button, run-mode entry, cron `agentType`, Docker and remote-SSH command defaults, and clone-repo Brain option.
+
+- **New resolver** `src/utils/pi-cli-resolver.ts`. Unlike the sibling resolvers it sanity-probes `pi --version` and requires semver-shaped output, because `pi` is a short generic name that a stray binary on `$PATH` can shadow; the rejected path is logged. `GET /api/pi/status` returns `{ available, path, version }` so a misresolution is diagnosable.
+- **`PiConfig`** maps to `--model` (accepts `provider/id` and a `:thinking` suffix), `--provider`, `--thinking`, `--session`/`-c`, and the tri-state `--approve` / `--no-approve`. Every value is regex-allowlisted and dropped on failure. `--api-key` is deliberately never wired: it would put a provider secret on the spawn command line.
+- **No bypass flag.** Pi has no permission prompts and no sandbox, so there is no `--dangerously-skip-permissions` analog. Its privilege-shaped knob is `approveProjectTrust`, which makes pi load and execute repo-local `.pi/extensions` TypeScript and install missing project packages. `clampExternalCliBypassForOwner()` therefore puts pi in the **materialize** branch: a non-granted multi-user owner gets `--no-approve` even when no config was sent, because pi's own default is an interactive prompt the session user could answer themselves. That helper had no test coverage at all; it now does, for all four CLIs.
+- **Env allowlist gains only the `PI_*` prefix.** Pi's ~34 provider key vars share no prefix and `ALLOWED_ENV_PREFIXES` is one global list with no mode context, so admitting them would widen the allowlist for every mode at once. Users authenticate via pi's `/login` or the server process's own environment.
+- **Pi stays out of `isAltScreenStripMode()`.** Its default TUI renders into the main screen with terminal-owned scrollback, and since 0.84.0 the user can flip to a fullscreen TUI at runtime via `/settings` — verified to switch the pane into the alt screen, which the strip would have corrupted.
+- **Docker**: pi installs in its own `--ignore-scripts` step so that flag cannot affect the other four CLIs, and its credentials are seeded per-file (`auth.json`, `settings.json`, `trust.json`, `models.json`, `models-store.json`) rather than whole-dir, since `~/.pi/agent` also holds sessions, extensions and installed package trees.
+- **Local echo**: pi lands on the buffer overlay. Verified that codex's per-keystroke starvation does not reproduce — pi's slash picker re-filters on the whole composer content, so a one-shot flush behaves identically to per-keystroke typing.
+- Installer detection, docs (`docs/pi-integration.md`), READMEs, and the architecture invariants are updated. Tests: `test/pi-mode.test.ts` and `test/routes/external-cli-bypass-clamp.test.ts`, plus extensions to the run-mode, mobile-overview, render-index-html, system-routes and local-echo suites.