feat(grok): add Grok Build (xAI) as a seventh CLI run mode

SessionMode gains 'grok', a first-class backend alongside Claude Code,
shell, OpenCode, Codex, Gemini, Antigravity and Pi: its own PTY, tmux
session, charcoal tab identity ('gk' badge), welcome button, run-mode
entry, cron agentType, Docker and remote-SSH command defaults, and
clone-repo Brain option. Flag surface verified live against grok 1.0.5.

Grok mixes two existing shapes and the wiring follows from that:

- Codex-shaped on permissions: the bypass switch is GrokConfig.alwaysApprove
  (--always-approve, grok's bypassPermissions mode; config-level deny rules
  still apply on top). The Run button sends it true, like runAntigravity(),
  and clampExternalCliBypassForOwner() puts grok in the only-if-sent branch:
  a bare grok spawn is grok's own ask-mode default, which is already safe,
  so only a sent config needs the flag forced off. Cron needs nothing for
  the same reason.
- OpenCode-shaped on rendering: grok is a fullscreen alternate-screen TUI
  with mouse support, so it stays OUT of isAltScreenStripMode() and lands
  on the narrow tmux-attach strip and the 'buffer' local-echo fallthrough
  (unmeasured against an authenticated composer; documented fallback is the
  'off' branch).
- Pi-shaped on resolution: 'grok' has npm squatters (@vibe-kit/grok-cli
  also installs a grok bin), so grok-cli-resolver.ts version-probes every
  candidate (grok --version, killSignal SIGKILL, VITEST-gated) and
  GET /api/grok/status surfaces path AND version; GROK_VERSION_REGEX is
  shared with the dependency registry so doctor and run mode cannot drift.

Env allowlist gains GROK_* plus the XAI_* vendor namespace (XAI_API_KEY is
grok's documented headless auth var), the same narrow-vendor reasoning as
GOOGLE_* for gemini. Resume is id-regexed on purpose: grok's own --resume
also matches session titles, which are arbitrary user strings that must
never reach the bash -c spawn line.

Docker: grok is not on npm, so the agent image installs it in its own step
(xAI's installer has no --dir override; the binary is copied to
/usr/local/bin and root's ~/.grok dropped in the same layer), and
credentials are seeded per-file (auth.json, config.toml, pager.toml; the
dir also holds sessions/, memory/ and the ~160MB binary). Remote SSH routes
through the login-shell wrapper like the other agent CLIs.

Verified end to end on an isolated CODEMAN_INSTANCE with grok 1.0.5
installed: /api/grok/status resolves and reports the probed version,
quick-start spawns a pane whose command line ends in 'grok
--always-approve', the real TUI renders (OAuth device screen on an
unauthenticated box), and grokConfig round-trips through state.json.
Docs: docs/grok-integration.md (user guide) + docs/grok-integration-plan.md
(decisions, verification record, follow-ups).

Tests: test/grok-mode.test.ts, test/grok-cli-resolver.test.ts, plus
extended clamp/system-routes/render-index-html/run-mode-ui/mobile-overview/
local-echo-gating coverage. npm test (the CI gate) green: 5910 tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docker/agent.Dockerfile

@@ -51,6 +51,17 @@ RUN npm install -g --ignore-scripts @earendil-works/pi-coding-agent \
  && npm cache clean --force \
  && pi --version
 
+# Grok Build (`grok`, xAI) is NOT on npm: a standalone ~160MB Rust binary through
+# xAI's installer, which targets $HOME/.grok/bin with no --dir override. At build
+# time that is root's home and unreachable by the `agent` user, so copy the binary
+# (through the bin/ symlink, hence -L) into /usr/local/bin and drop root's ~/.grok
+# in the same layer so the image does not carry the download twice.
+RUN curl -fsSL https://x.ai/cli/install.sh | bash \
+ && cp -L /root/.grok/bin/grok /usr/local/bin/grok \
+ && chmod 755 /usr/local/bin/grok \
+ && rm -rf /root/.grok /root/.local/bin/grok /root/.local/bin/agent \
+ && grok --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
@@ -68,11 +79,12 @@ 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
+# `.pi/agent` and `.grok` ARE pre-created: both are seeded per-FILE (pi:
+# auth/settings/trust/models; grok: auth.json/config.toml/pager.toml), 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/.pi/agent \
+      /home/agent/.claude/projects /home/agent/.codex/sessions /home/agent/.pi/agent /home/agent/.grok \
  && chgrp -R 0 /home/agent \
  && chmod -R g=u /home/agent