Antigravity CLI provider: explicit tier defaults, instant empty-completion resample, doc truth-up

[andreBurnt]

Implements the council-reviewed remediation for LincForge/LINC#571 (Antigravity CLI latency divergence). Design spec + measurements: 2026-09-01-agy-latency-remediation-design.md.

Changes

• Explicit effort-suffixed tier defaults (AGY_TIER_MODELS): fast=flash-low, balanced=flash-medium, smart/code=flash-high (deliberately shared — no higher non-pro rung; commented). auto stays user-selectable. Tier-routed calls resolve through the map (mirrors GeminiProvider).
• Instant resample on empty completions: agyRetryDelayMs returns 0 for EmptyCompletionError (stochastic model abandonment, not load-shedding) — drops up to 7s of dead backoff from the 25s+ outlier band; transient CLI errors keep exponential backoff. Wired via a new backward-compatible RetryOptions.delayMs hook.
• Registry staleness tripwire: agy-gated test diffs AGY_MODELS against live agy models; it fired on its first run — the gemini-3.5-flash-* trio is already sunset upstream. Removed, with modelMigrations mapping saved 3.5 routing to the current 3.7 line at the same effort.
• Header truth-up: measured catalog cost (57 tools / ~24.2k tokens, never cache-hit cross-request), no isolation mechanism in agy 1.1.23 (HOME-override rejected — forks OAuth state), allow-rules confirmed live-executing (run_command, write_to_file).

Not done (deliberate, per spec)

• Warm PTY transport: deferred — floor is prefill-dominated; PTY saves only boot (~0.5–1s of ~2.8s).
• Skill/config isolation: closed as no-mechanism.
• Direct-gemini promotion: already selectable via stored credential; no code change.

Downstream impact

• RetryOptions (changed): new optional delayMs field — additive, no consumer breaks.
• AGY_TIER_MODELS, agyRetryDelayMs (new exports).
• describe().defaultTierModels values (changed): the built-in antigravity-cli Settings preset now routes tiers to explicit models instead of auto. Saved user routings untouched except sunset 3.5 ids, which migrate to 3.7 via the existing modelMigrations path (codex precedent).

Verification

• pnpm tsx --test 'src/**/*.test.ts' → 12/12 pass (3 pre-existing + 9 new), npx tsc --noEmit clean.
• No CI test gate on this repo (release.yml only) — local suite is the gate.
• Live bench: all four tier model strings verified accepted through agy 1.1.23 (~3s, SUCCESS).

[andreBurnt]

Superseded — wrong target: per the house convention this goes upstream to mempko/abject (like mempko#10/mempko#11/mempko#12). Re-opening there after a maintainer-persona council pass; branch restructured to the no-tests-in-tree convention.