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.

docs/architecture/decisions/004-frontmatter-field-normalization.md

@@ -0,0 +1,55 @@
+---
+status: stable
+last_updated: 2026-04-28
+---
+
+# ADR-004: Frontmatter Field Name Normalization (depends_on / dependsOn)
+
+## Context
+
+There was a naming divergence between the Rust CLI and the TypeScript core library for the dependency field:
+
+| Source | Field name in YAML | Field name in struct |
+|--------|--------------------|---------------------|
+| Rust CLI (`taskgraph`) | `depends_on` | `depends_on` |
+| TypeScript lib (`@alkdev/taskgraph`) | `dependsOn` | `dependsOn` |
+
+The `yaml` npm package does **not** auto-convert snake_case to camelCase. A markdown file with `depends_on: [a, b]` would parse to `{depends_on: ["a", "b"]}`, which the `TaskInput` schema (expecting `dependsOn`) rejected as an unknown property. `Value.Clean()` would strip it, and `Value.Check()` would fail because the required field was missing.
+
+This was a bug in `@alkdev/taskgraph`. The library's `parseFrontmatter()` function contract says it accepts "markdown with YAML frontmatter" — but the YAML convention established by the Rust CLI ecosystem was `depends_on`, and the parser silently discarded it.
+
+**Broader point**: This was a textbook example of how issues upstream increase the surface area of issues downstream. A field naming convention in the Rust implementation created a compatibility fault line that propagated to every consumer. These are the "corners" that are hard to see around in linear text — exactly the kind of problem DAG-structured task analysis is designed to surface.
+
+## Decision
+
+**Fixed upstream in `@alkdev/taskgraph` v0.0.2**: A normalization step was added to `parseFrontmatter()` between YAML parsing and `Value.Clean()`. Known snake_case aliases are mapped to their camelCase canonical names.
+
+The normalization map:
+
+```typescript
+const KEY_ALIASES: Record<string, string> = {
+  depends_on: "dependsOn",
+}
+```
+
+Applied after YAML parse, before `Value.Clean()`. Both `depends_on` and `dependsOn` are now accepted in YAML frontmatter. The canonical form for new files is `dependsOn` (camelCase).
+
+## Resolution
+
+- `@alkdev/taskgraph` v0.0.2 includes the fix
+- This plugin pins `^0.0.2` in its dependencies
+- No plugin-level workaround needed
+- The `depends_on` / `dependsOn` compatibility surface is resolved
+
+## Impact on This Plugin
+
+**Resolved**. Task files using either `depends_on` (Rust CLI convention) or `dependsOn` (TypeScript canonical) parse correctly. No preprocessing, workarounds, or special handling required in the plugin.
+
+AGENTS.md documents `dependsOn` as the canonical form for new task files, with a note that both forms are accepted.
+
+## References
+
+- Rust CLI `struct TaskFrontmatter`: uses `depends_on` (snake_case, Serde default)
+- TypeScript `TaskInput` schema: uses `dependsOn` (camelCase, JS convention)
+- `yaml` npm package: preserves YAML key casing as-is (no auto-conversion)
+- `Value.Clean()`: previously stripped `depends_on` as unknown property — now handled by normalization upstream