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/003-risk-merge.md

@@ -0,0 +1,44 @@
+---
+status: draft
+last_updated: 2026-04-28
+---
+
+# ADR-003: Merged `risk` Operation (Risk Path + Risk Distribution)
+
+## Context
+
+The taskgraph CLI exposes two separate risk-related subcommands:
+- `taskgraph risk` — shows risk distribution (tasks grouped by risk level: trivial, low, medium, high, critical)
+- `taskgraph risk-path` — shows the single highest-cumulative-risk path through the DAG
+
+Both are about understanding risk in the task graph. An agent asking "what's the risk situation?" almost always wants both perspectives — which tasks are risky, and where does risk concentrate along paths.
+
+## Decision
+
+Merge into a single `risk` operation that returns:
+1. **Risk distribution** — tasks grouped by risk level (trivial → critical), with counts and percentages
+2. **Highest risk path** — the path through the DAG with maximum cumulative risk, showing per-task risk and impact
+
+This maps to `riskDistribution(graph)` and `riskPath(graph)` from `@alkdev/taskgraph`.
+
+## Consequences
+
+**Positive:**
+- One call gives the complete risk picture
+- Agent doesn't need to correlate results from two separate calls
+- The distribution provides context for understanding the risk path (e.g., "3 high-risk tasks, 2 of which are on the critical path")
+
+**Negative:**
+- Output is larger than individual calls
+- An agent that only wants distribution or only wants the path gets extra content
+- Slightly more complex formatting logic
+
+**Mitigation for negatives:**
+- The combined output is still well under typical markdown rendering limits
+- Distribution is shown first (most likely to be actioned on), path second (deeper analysis)
+- Both sections have clear headers so the agent can focus on what matters
+
+## References
+
+- taskgraph CLI: `taskgraph risk` and `taskgraph risk-path` subcommands
+- `@alkdev/taskgraph`: `riskDistribution()` and `riskPath()` functions