feat(agentstack): add @logicsrc/agentstack coordination module (#5)

* 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>

* docs(agentstack): note dual-runtime support (Bun + Node.js + Workers + browser)

Matches the revised Shared AppKit PRD's Runtime Compatibility Strategy.
AgentStack is runtime-neutral (no Bun/Node-only APIs; time is injected).

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

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

packages/agentstack/src/manifest.ts

@@ -0,0 +1,22 @@
+import type { PluginManifest } from "@logicsrc/plugin-core";
+
+export const agentStackManifest: PluginManifest = {
+  id: "agentstack",
+  name: "AgentStack",
+  version: "0.1.0",
+  type: ["agents", "tasks", "coordination"],
+  default: false,
+  capabilities: [
+    "agents.register",
+    "agents.delegate",
+    "tasks.create",
+    "tasks.assign",
+    "tasks.update",
+    "tasks.publish",
+    "reputation.sync",
+    "payments.link",
+    "escrow.link"
+  ],
+  commands: ["agents", "tasks", "delegate"],
+  env: ["AGENTSTACK_API_URL", "AGENTSTACK_API_KEY", "COINPAY_API_BASE_URL"]
+};