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>

test/render-index-html.test.ts

@@ -17,6 +17,7 @@ import { isOpenCodeAvailable } from '../src/utils/opencode-cli-resolver.js';
 import { isCodexAvailable } from '../src/utils/codex-cli-resolver.js';
 import { isGeminiAvailable } from '../src/utils/gemini-cli-resolver.js';
 import { isAntigravityAvailable } from '../src/utils/antigravity-cli-resolver.js';
+import { isPiAvailable } from '../src/utils/pi-cli-resolver.js';
 import { isCloudflaredAvailable } from '../src/utils/cloudflared-resolver.js';
 import { isGitAvailable } from '../src/git-clone.js';
 
@@ -43,6 +44,11 @@ vi.mock('../src/utils/antigravity-cli-resolver.js', () => ({
   isAntigravityAvailable: vi.fn(() => false),
   resolveAntigravityDir: vi.fn(() => null),
 }));
+vi.mock('../src/utils/pi-cli-resolver.js', () => ({
+  isPiAvailable: vi.fn(() => false),
+  resolvePiDir: vi.fn(() => null),
+  getPiCliVersion: vi.fn(() => null),
+}));
 vi.mock('../src/utils/cloudflared-resolver.js', () => ({
   isCloudflaredAvailable: vi.fn(() => false),
   resolveCloudflaredPath: vi.fn(() => null),
@@ -131,6 +137,7 @@ describe('WebServer.renderIndexHtml', () => {
     vi.mocked(isCodexAvailable).mockReturnValue(true);
     vi.mocked(isGeminiAvailable).mockReturnValue(false);
     vi.mocked(isAntigravityAvailable).mockReturnValue(false);
+    vi.mocked(isPiAvailable).mockReturnValue(true);
     vi.mocked(isCloudflaredAvailable).mockReturnValue(true);
     vi.mocked(isGitAvailable).mockReturnValue(true);
     const { server } = makeServer({});
@@ -144,6 +151,7 @@ describe('WebServer.renderIndexHtml', () => {
       codex: true,
       gemini: false,
       antigravity: false,
+      pi: true,
       cloudflared: true,
       git: true,
     });
@@ -158,6 +166,7 @@ describe('WebServer.renderIndexHtml', () => {
       isCodexAvailable,
       isGeminiAvailable,
       isAntigravityAvailable,
+      isPiAvailable,
       isCloudflaredAvailable,
       isGitAvailable,
     ]) {