Decompose monolithic architecture.md into modular docs/architecture/ documents

The 751-line architecture.md violated the SDD process modular documentation
target (~500 lines). It also had duplicate TaskGraph class definitions (one
monolith, one decomposed) that directly contradicted each other, and embedded
consumer-specific tool dispatch mappings that belong in downstream projects.

Changes:
- Split into 8 focused documents + 7 ADR records + redirect page
- Removed the monolithic TaskGraph class (kept only decomposed version)
- Moved CLI→plugin dispatch mapping out (belongs in plugin architecture)
- Extracted implementation code (frontmatter splitter, findCycles, DAG
  propagation) into WHAT/WHY descriptions per architect role spec
- Added proper ADR format for all resolved design decisions
- Fixed review issues: C_fail mapping, DuplicateNodeError/DuplicateEdgeError
  types, ValidationError/GraphValidationError definitions, mutation error
  handling contract, enum naming convention, validation timing clarification

docs/architecture/decisions/003-topo-order-throws-on-cycle.md

@@ -0,0 +1,27 @@
+# ADR-003: topologicalOrder throws CircularDependencyError on cyclic graphs
+
+**Status**: Accepted
+
+## Context
+
+When a graph has cycles, topological sort cannot produce a complete ordering. Options: return `null`, return a partial ordering, or throw an error with cycle information.
+
+## Decision
+
+**Throw `CircularDependencyError`** with `cycles` populated from `findCycles()`. Do not return a partial ordering or `null`.
+
+## Consequences
+
+### Positive
+- Prevents silent ignoring of cycles — consumers get explicit error information
+- `CircularDependencyError.cycles` provides the actual cycle paths for error reporting
+- Simpler return type — `string[]` instead of `string[] | null` or `string[][]`
+- Both consumers treat cycles as bugs: alkhub data comes from validated DB schema; OpenCode plugin data comes from frontmatter that should be validated before graph construction
+
+### Negative
+- Callers who want "best effort" ordering on cyclic graphs must catch the error and call `findCycles()` separately
+- Cannot get partial results — if you want "topo sort of the acyclic portions," that requires filtering first
+
+### Mitigation
+
+`findCycles()` and `hasCycles()` are available for consumers that want to handle cycles gracefully before calling `topologicalOrder()`.