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>

plugins/agentgit/src/access.ts

@@ -0,0 +1,51 @@
+import type { MemberRole, Repo } from "./domain.js";
+
+export type Action = "read" | "write" | "review" | "merge" | "admin";
+
+export interface GateResult {
+  allowed: boolean;
+  role?: MemberRole;
+  reason?: string;
+}
+
+const ROLE_ACTIONS: Record<MemberRole, Action[]> = {
+  reader: ["read"],
+  reviewer: ["read", "review"],
+  contributor: ["read", "review", "write"],
+  maintainer: ["read", "review", "write", "merge", "admin"]
+};
+
+function rolePermits(role: MemberRole, action: Action): boolean {
+  return ROLE_ACTIONS[role].includes(action);
+}
+
+/**
+ * The membership gate. Every AgentGit operation runs through this before any
+ * backend call. The owner is an implicit maintainer; public repos allow `read`
+ * to anyone; everything else requires a matching member role. There is no
+ * anonymous access to members_only/private repos.
+ */
+export function gateAccess(repo: Repo, callerDid: string | undefined, action: Action): GateResult {
+  if (callerDid && callerDid === repo.owner_did) {
+    return { allowed: true, role: "maintainer" };
+  }
+
+  if (action === "read" && repo.visibility === "public") {
+    return { allowed: true, role: "reader" };
+  }
+
+  if (!callerDid) {
+    return { allowed: false, reason: "authentication required (DID)" };
+  }
+
+  const member = repo.members.find((entry) => entry.did === callerDid);
+  if (!member) {
+    return { allowed: false, reason: `${callerDid} is not a member of ${repo.slug}` };
+  }
+
+  if (!rolePermits(member.role, action)) {
+    return { allowed: false, role: member.role, reason: `role "${member.role}" cannot ${action}` };
+  }
+
+  return { allowed: true, role: member.role };
+}