feat: make Antigravity a first-class CLI across docs, installer and UI

Antigravity (agy) was wired into the session layer but never propagated to
the surfaces around it, while Gemini CLI stayed documented as a consumer
product despite being enterprise-only since Google's cutover. Gemini keeps
full support; Antigravity now sits beside it everywhere.

Functional fixes:
- docker/agent.Dockerfile never installed agy, so a docker case with
  mode 'antigravity' died on command-not-found. agy is not on npm, so it
  gets its own installer step. --dir /usr/local/bin is load-bearing: the
  default $HOME/.local/bin resolves to root's home at build time and is
  unreachable by the `agent` user the container runs as. Verified inside
  codeman/agent:base (v1.1.10, reachable as `agent`). Note the binary is
  ~190MB, the largest layer in the image.
- Welcome screen gained a Run Antigravity action, gated on agy being
  present like the other CLI buttons, with a cyan identity matching the
  toolbar run button and run-mode dot.
- install.sh now detects agy (search paths mirroring the resolver), counts
  it as a satisfying AI CLI, and recommends it over Gemini in the install
  hints. Detection only, no new auto-install path.

Docs corrected where they were factually wrong:
- architecture-invariants documented isExternalCliMode() as
  opencode/codex/gemini when the code has included antigravity for a
  while, said "all three modes", and omitted ANTIGRAVITY_ from the env
  prefix allowlist row.
- cron-guide's agentType enum, cron-discovery's SessionMode, and
  remote-sessions' RemoteCommandMode were all stale.

Also: README + README.zh-CN (five CLIs, Gemini marked enterprise-only),
package.json keyword, and comment drift in 8 places.

test/run-mode-ui.test.ts now covers the new welcome button; verified it
fails without the settings-ui wiring.

Antigravity nests its whole state under ~/.gemini/antigravity-cli/, not
~/.antigravity, so the existing .gemini docker credential seed already
covers it. Recorded as a comment so nobody adds dead config later.

isAltScreenStripMode() deliberately still excludes antigravity: whether
its TUI needs the alt-screen strip is a behavioural question that needs a
real agy session, not a guess.

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

src/web/public/app.js

@@ -1976,7 +1976,7 @@ class CodemanApp {
       // Source 2: Terminal buffer fallback — strip ANSI, drop Claude CLI chrome.
       // Claude + shell only: _cleanTerminalBuffer knows Claude CLI's output, and
       // shell sessions have no transcript source at all; for TUI modes
-      // (codex/opencode/gemini) it yields repaint garbage, so a clear
+      // (codex/opencode/gemini/antigravity) it yields repaint garbage, so a clear
       // placeholder beats a messy screen dump there.
       const sessionMode = this.sessions.get(this.activeSessionId)?.mode || 'claude';
       if (!lastResponse && (sessionMode === 'claude' || sessionMode === 'shell')) {
@@ -4281,7 +4281,7 @@ class CodemanApp {
       // (viewport + scrollback + colors) for an instant first paint. For codex
       // this is also a correctness fix — its byte-stream replay shows only the
       // latest TUI frame (the idle welcome banner) because codex doesn't include
-      // earlier conversation in its current redraw. For claude/opencode/gemini
+      // earlier conversation in its current redraw. For claude/opencode/gemini/antigravity
       // the replay is already complete, so the snapshot is purely a faster,
       // scroll-preserving first paint before the canonical fetch reconciles.
       //

src/web/public/index.html

@@ -323,6 +323,10 @@
               <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polygon points="5 3 19 12 5 21 5 3"/></svg>
               Run OpenCode
             </button>
+            <button class="welcome-btn welcome-btn-antigravity" id="welcomeAntigravityBtn" style="display: none;" onclick="app.setRunMode('antigravity'); app.runAntigravity()">
+              <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polygon points="5 3 19 12 5 21 5 3"/></svg>
+              Run Antigravity
+            </button>
             <button class="welcome-btn welcome-btn-gemini" id="welcomeGeminiBtn" style="display: none;" onclick="app.setRunMode('gemini'); app.runGemini()">
               <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><polygon points="5 3 19 12 5 21 5 3"/></svg>
               Run Gemini
@@ -2192,7 +2196,7 @@
             <div class="form-row">
               <label>Image</label>
               <input type="text" id="dockerImage" placeholder="codeman/agent:base" autocomplete="off" autocapitalize="off" spellcheck="false">
-              <span class="form-hint">Build it once with <code>node scripts/build-agent-image.mjs</code>. Contains node + claude/codex/gemini + tmux.</span>
+              <span class="form-hint">Build it once with <code>node scripts/build-agent-image.mjs</code>. Contains node + claude/codex/gemini/opencode/agy + tmux.</span>
             </div>
             <div class="form-row">
               <label>Network</label>

src/web/public/session-ui.js

@@ -1,5 +1,5 @@
 /**
- * @fileoverview Quick start (case loading, session spawning for Claude/Shell/OpenCode/Codex/Gemini),
+ * @fileoverview Quick start (case loading, session spawning for Claude/Shell/OpenCode/Codex/Gemini/Antigravity),
  * session options modal (per-session settings, color picker, rename),
  * session options tabs (Ralph config tab), case settings (CRUD, links),
  * create case modal, and mobile case picker.

src/web/public/settings-ui.js

@@ -748,6 +748,7 @@ Object.assign(CodemanApp.prototype, {
     const buttons = [
       ['welcomeClaudeBtn', 'claude'],
       ['welcomeOpencodeBtn', 'opencode'],
+      ['welcomeAntigravityBtn', 'antigravity'],
       ['welcomeGeminiBtn', 'gemini'],
       // Not a run mode, same reasoning: offering a Cloudflare Tunnel on a box
       // without cloudflared can only ever produce "cloudflared not found".

src/web/public/styles.css

@@ -3305,6 +3305,23 @@ body.touch-device .terminal-container .xterm .xterm-helper-textarea {
   transform: translateY(-1px);
 }
 
+/* Antigravity: cyan identity, matching .btn-toolbar.btn-run.mode-antigravity and
+   .run-mode-dot.antigravity so the welcome action reads as the same backend. */
+.welcome-btn-antigravity {
+  background: linear-gradient(135deg, #0b2b33 0%, #0e7490 55%, #0891b2 100%);
+  border-color: rgba(34, 211, 238, 0.4);
+  color: #cffafe;
+  box-shadow: 0 2px 8px rgba(34, 211, 238, 0.16), inset 0 1px 0 rgba(255, 255, 255, 0.06);
+}
+
+.welcome-btn-antigravity:hover {
+  background: linear-gradient(135deg, #124450 0%, #0891b2 55%, #06b6d4 100%);
+  box-shadow: 0 4px 20px rgba(34, 211, 238, 0.3), 0 0 40px rgba(8, 145, 178, 0.12), inset 0 1px 0 rgba(255, 255, 255, 0.08);
+  border-color: rgba(103, 232, 249, 0.5);
+  color: #ecfeff;
+  transform: translateY(-1px);
+}
+
 .welcome-btn-gemini {
   background: linear-gradient(135deg, #10243f 0%, #174ea6 55%, #4f46e5 100%);
   border-color: rgba(96, 165, 250, 0.4);

src/web/public/terminal-ui.js

@@ -1408,7 +1408,7 @@ Object.assign(CodemanApp.prototype, {
     }
     titleSpan.appendChild(document.createTextNode(s.name || s.firstPrompt || shortDir));
 
-    // Badge row: mode (claude/codex/opencode/gemini/shell) + a LIVE pill.
+    // Badge row: mode (claude/codex/opencode/gemini/antigravity/shell) + a LIVE pill.
     const badgeRow = document.createElement('div');
     badgeRow.className = 'history-item-badges';
     if (s.mode) {