feat(agentstack): add @logicsrc/agentstack coordination module

AgentStack is the LogicSRC module for portable agent, task, payment, and
reputation coordination across Profullstack apps. Reference implementation of
the `agentstack` capability in the Profullstack Shared AppKit OpenSpec.

- packages/agentstack: DID helpers (did:coinpay:user/agent), DidTask model +
  lifecycle, AgentStack in-memory coordinator (agents, tasks, delegation,
  events), and a validated LogicSRC plugin definition. 9 vitest cases.
- Wire @logicsrc/agentstack into the root build chain (after plugin-core).
- docs/agentstack.md + mention in openspec-comparison.md.

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

packages/agentstack/tsconfig.json

@@ -0,0 +1,8 @@
+{
+  "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "rootDir": "src",
+    "outDir": "dist"
+  },
+  "include": ["src/**/*.ts"]
+}