Add architecture specification and bump taskgraph to v0.0.2

Architecture docs for the open-tasks plugin covering the registry pattern
dispatch design, operation set, error handling, data flow, and constraints.
Includes four ADRs (registry pattern, no-cache policy, risk operation merge,
frontmatter normalization). The depends_on/dependsOn compatibility issue in
@alkdev/taskgraph is resolved in v0.0.2, so the dependency is bumped and
the docs reflect the fix.

AGENTS.md updated: canonical dependsOn field, dependents operation added,
hooks clarification, field naming note.

package.json

@@ -41,7 +41,7 @@
     "decomposition"
   ],
   "dependencies": {
-    "@alkdev/taskgraph": "^0.0.1",
+    "@alkdev/taskgraph": "^0.0.2",
     "@opencode-ai/plugin": "^1.1.3"
   },
   "devDependencies": {