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.
       //