feat(providers): add the Antigravity setup surfaces

[rynfar]

Antigravity has been reachable only from the server since #425. This adds the
web and mobile surfaces that let a user actually sign in and use it: a setup
section in provider settings, a mobile setup screen reachable from both
connection entry points, the provider's icon and picker entries, and user docs.

Stacked on #433, which adds the setup RPCs this UI binds to.

What changed

Web. Antigravity instances get a setup section above their runtime settings,
covering all four sign-in methods the schema declares — Google account, Gemini
Enterprise, Gemini API key, and Agent Platform. Browser methods hand off to
Google and wait for the callback; credential methods verify what is configured
in the instance settings. Antigravity joins the model picker, provider settings
list, and settings search.

Mobile. The same flow as a route, reachable from Settings → Environments and
from Connections. Antigravity's icon ships as an asset.

Docs. docs/user/providers-antigravity.md, plus the provider and install
guides.

Adopted from T3 Code

Ported manually from 06336460c9988f29c71e839c4c9c840c4552e077 (#9348).

Applying that commit directly was rejected: it predates several Pylon changes and
its diff reverts them. Replaying it conflicts in 36 files across 119 hunks, and
most of those conflicts are not about Antigravity at all — it restores
LoaderIcon over the Spinner from #426, swaps
getBackgroundTextGenerationProviders back to textGenerationProviders, returns
per-instance provider updates to per-driver, and deletes Pylon's provider
unavailable banner handling and rollbackMutationBlocked. The Antigravity
additions were extracted and applied to Pylon's current code instead.

Pylon adaptations

• Prime survives. The upstream diff presents Pylon's Prime Agent entry as a
  deletion replaced by Antigravity in all four provider lists — icons, driver
  metadata, session provider options, and the picker. Both are present here.
• No tabbed card. Upstream renders its setup surface inside a tabbed
  ProviderInstanceCard. Pylon's card is a flat sectioned layout, so the new
  setup slot renders above the Runtime section instead.
• Settings patch shape. AntigravitySettingsPatch takes
  Array(CustomModelSetting) to match Pylon's custom model entries from feat: add custom model names and option descriptors #423,
  not upstream's Array(String). Without the patch entry, saving Antigravity
  auth settings fails at the wire even though the full settings schema accepts
  the field.
• Both mobile entry points. Upstream reaches the setup screen from its
  connection rows; Pylon renders that row from Settings → Environments as well,
  so both navigate to it.
• Version formatting. Antigravity reports release tags such as
  agy_acp_server_20260818_01_RC01, which is not semver. Those render as a date
  and candidate, and the v prefix now applies only to bare numeric versions.

Verification

• Tests: 108 across the new setup section, mobile setup state, and session logic.
• Types: all seven packages clean.
• vp check: 0 errors across 3360 files.
• Unused exports and files: knip:check clean.

ANTIGRAVITY_AUTH_METHODS is exported here rather than in #433 so the export
lands with the consumer that needs it.

Not yet verified against a live Antigravity account; the flows above are covered
by unit and component tests only.

Model and harness: Claude Opus 5 via Claude Code.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.8 KiB	13.7 KiB	−170 B (−1.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	−1 B (−0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.6 KiB	−169 B (−2.5%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.4 KiB	56.5 KiB	−910 B (−1.5%)	66.4 KiB	✅
Codex	Live turn messages	10	8	−2 (−20.0%)	21	✅
Claude	Total thread wire	13.8 KiB	13.7 KiB	−156 B (−1.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+6 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.6 KiB	−162 B (−2.4%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.2 KiB	57.3 KiB	−928 B (−1.6%)	66.4 KiB	✅
Claude	Live turn messages	10	8	−2 (−20.0%)	21	✅

Baseline: f6bf292 · PR result: cb48a88 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 111.2 KiB
• Claude decoded thread snapshot: 111.9 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.