refactor: delegate review and PR gating to Pi plugins

[BakerSean168]

Summary

• remove ForgeFlow's duplicate forgeflow.review and forgeflow.accept workflow runtimes
• remove their GitHub/reviewer helper scripts and dedicated tests
• keep ForgeFlow focused on logical model roles, deterministic invariant policy, one-writer governance, and native-child extension registration
• document pi-subagents as the owner of reviewer/acceptance primitives and pi-gauntlet/gatekeep-pr as the exact-head PR gate when installed
• add an architecture boundary preventing the duplicate review/PR runtime from returning

Why

The installed Pi ecosystem already owns these capabilities:

• pi-subagents: reviewers, review loops, typed gates, runtime acceptance evidence
• pi-gauntlet/gatekeep-pr: exact-head CI evidence, head freshness, review orchestration, compare-and-swap, and --match-head-commit merge safety

Maintaining a second ForgeFlow implementation created two sources of truth for the same engineering gate.

Verification

• npm run check: 16/16 passed
• npm pack --dry-run: passed; package now contains 9 files / 30.6 kB unpacked
• git diff --check: passed
• runtime surface: extension/index.js, extension/invariants.js, extension/model-policy.js
• net diff: 1531 lines deleted, 133 lines added/rewritten