Add AgentGit M1: agent-native git layer over a Forgejo backend

AgentGit is a thin, DID-gated source-collaboration layer over a backend
forge (default Forgejo at git.profullstack.com, BBS-members-only) — not a
new git host. M1 implements the contract and engines:

- forge/adapter.ts: ForgeAdapter interface (only forge-specific surface)
- forge/forgejo.ts: ForgejoAdapter over Forgejo/Gitea REST v1 (injectable
  fetch, typed errors), incl. ensureUser for member provisioning
- access.ts: gateAccess DID membership gate (owner/role/visibility)
- merge-policy.ts: evaluateMergePolicy pure engine (reviews, reputation
  floor, checks, escrow, merge method, agent-merge toggle)
- service.ts: AgentGitService ties gate + policy to the adapter; refuses
  policy-failing merges; provisionMember hook for AgentBBS
- schemas: logicsrc-repo + logicsrc-pull-request, registered in
  @logicsrc/validators with fixtures
- docs/agentgit.md spec; plugin wired into root build (default/disabled)

27 vitest tests pass; full monorepo build green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packages/validators/package.json

@@ -11,7 +11,7 @@
   "scripts": {
     "build": "tsc -p tsconfig.json",
     "test": "vitest run src",
-    "validate:fixtures": "node dist/cli.js task ../schemas/fixtures/task.yaml && node dist/cli.js agent ../schemas/fixtures/agent.yaml && node dist/cli.js agentad-ad ../schemas/fixtures/agentad-ad.yaml && node dist/cli.js agentad-placement ../schemas/fixtures/agentad-placement.yaml"
+    "validate:fixtures": "node dist/cli.js task ../schemas/fixtures/task.yaml && node dist/cli.js agent ../schemas/fixtures/agent.yaml && node dist/cli.js agentad-ad ../schemas/fixtures/agentad-ad.yaml && node dist/cli.js agentad-placement ../schemas/fixtures/agentad-placement.yaml && node dist/cli.js repo ../schemas/fixtures/repo.yaml && node dist/cli.js pull-request ../schemas/fixtures/pull-request.yaml"
   },
   "dependencies": {
     "ajv": "^8.17.1",