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>

package-lock.json

@@ -1430,6 +1430,10 @@
       "resolved": "packages/account-core",
       "link": true
     },
+    "node_modules/@logicsrc/agentstack": {
+      "resolved": "packages/agentstack",
+      "link": true
+    },
     "node_modules/@logicsrc/cli": {
       "resolved": "packages/cli",
       "link": true
@@ -6063,6 +6067,16 @@
         "vitest": "^4.0.8"
       }
     },
+    "packages/agentstack": {
+      "name": "@logicsrc/agentstack",
+      "version": "0.1.0",
+      "dependencies": {
+        "@logicsrc/plugin-core": "file:../../packages/plugin-core"
+      },
+      "devDependencies": {
+        "vitest": "^4.0.8"
+      }
+    },
     "packages/cli": {
       "name": "@logicsrc/cli",
       "version": "0.1.0",