D1007 — Antigravity receives SessionStart native hook and session bootstrap parity

Decision. Antigravity's hook registration mechanism in internal/provisioning/hooksettings.go registers SessionStart hooks, and hookEventReach declares that SessionStart reaches configurator.ProviderAntigravity. noSessionStartEvent is removed from Antigravity's hookMechanism in internal/provisioning/bootstrap.go, allowing EnsureBootstrapHook to install and manage the synthetic cartographer-bootstrap hook for Antigravity.

This amends D194 §Trigger semantics, which previously marked Antigravity with noSessionStartEvent: true when Antigravity only exposed per-tool and per-invocation events.

Why. Recent versions of Google Antigravity (agy and Antigravity 2.0) support the SessionStart event in hooks.json (https://antigravity.google/docs/hooks/). The event fires once at the beginning of a conversation session and expects a flat array of command handler objects, identical to Antigravity's PreInvocation and Stop handlers.

registerAntigravityHook already emits flat arrays for lifecycle events (all non-tool events other than PreToolUse and PostToolUse), so the registration handler requires no structural change: adding "SessionStart": true to antigravityHookEvents immediately yields valid Antigravity hook configurations. Enabling SessionStart and the bootstrap hook provides automatic, transparent session-start synchronization for Antigravity users, achieving parity with Claude Code, Codex, and Kiro.

Alternatives rejected. - Keep Antigravity on scheduled timer sync only. Now that Antigravity natively supports SessionStart, relying solely on external background cron timers or manual sync is unnecessary and leaves Antigravity with worse UX than other providers. - Use PreInvocation for bootstrap. Firing on every model invocation would cause continuous redundant sync runs on every turn, which is wasteful and alters the semantics.

Consequences. - SupportsSessionHook(ProviderAntigravity) returns true. - EnsureBootstrapHook installs .gemini/config/hooks/cartographer-bootstrap/ and registers cartographer-cartographer-bootstrap under SessionStart in .gemini/config/hooks.json. - cartographer connect antigravity and cartographer sync no longer report Antigravity as a hookless provider needing the scheduled sync timer. - cartographer disconnect antigravity cleanly prunes the bootstrap hook and deletes hooks.json when empty, preserving non-empty user configs.