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>

docker/agent.Dockerfile

@@ -44,6 +44,13 @@ RUN curl -fsSL https://antigravity.google/cli/install.sh | bash -s -- --dir /usr
  && chmod 755 /usr/local/bin/agy \
  && agy --version
 
+# Pi (pi.dev). Upstream documents --ignore-scripts (pi needs no lifecycle scripts);
+# kept out of the shared npm block above so the flag cannot silently change how the
+# other four CLIs install.
+RUN npm install -g --ignore-scripts @earendil-works/pi-coding-agent \
+ && npm cache clean --force \
+ && pi --version
+
 # `agent` user (gid 0) with an arbitrary-uid-writable HOME. The uid is
 # auto-assigned (node:22-slim already occupies uid 1000 with its `node` user); at
 # runtime Codeman overrides with `--user <hostUid>:0` on Linux, so the baked uid
@@ -61,9 +68,11 @@ ENV HOME=/home/agent
 # transcript/rollout dirs (`.claude/projects`, `.codex/sessions`) are bind-mounted from
 # the host. (gemini/gcloud/opencode are whole seed-copies and need no pre-created dir;
 # Antigravity nests its state inside `.gemini/antigravity-cli`, so it rides that seed.)
+# `.pi/agent` IS pre-created: pi is seeded per-FILE (auth/settings/trust/models), and a
+# per-file seed copy, unlike a whole-dir one, does not create its parent directory.
 RUN useradd -g 0 -m -d /home/agent -s /bin/bash agent \
  && mkdir -p /home/agent/.npm /home/agent/.cache /home/agent/.config /home/agent/.codeman \
-      /home/agent/.claude/projects /home/agent/.codex/sessions \
+      /home/agent/.claude/projects /home/agent/.codex/sessions /home/agent/.pi/agent \
  && chgrp -R 0 /home/agent \
  && chmod -R g=u /home/agent