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.
