feat: ship ForgeFlow v3 routing and Antigravity bridge

[BakerSean168]

Summary

• add Antigravity external-agent bridge owned by pi-subagents
• add adaptive thinking envelopes and deterministic multi-model routes
• add v3 supply groups separating logical capability from quota source
• prefer Business Team openai-codex/gpt-6.1-sol for the worker frontier route with narrow supply-only failover
• add usage/route telemetry without secrets

Runtime acceptance

• GCP runtime is already pinned to commit 214b1a4
• final T3 → Pi → ForgeFlow worker acceptance completed successfully
• worker route decision selected business-team, priority 10, physical model openai-codex/gpt-6.1-sol, failover count 0
• child worker returned successfully through a T3-owned Pi session

Validation

• npm run check: 40/40 tests pass
• repository/worktree clean before PR

Package version is already 3.0.0-alpha.1; after merge this should be tagged/released as the matching prerelease rather than leaving production pinned to an unmerged feature commit.