feat(T3.3): implement task dependency graph

- Add internal/graph package for dependency management
- Implement DependencyGraph for task ordering
- Support task dependencies and prerequisite tracking
- Validate graph for cycles (no circular dependencies)
- Topological sort for execution order (Kahn's algorithm)
- Track task status (pending, completed, failed)
- Get ready-to-execute tasks based on dependencies
- Get tasks that depend on a given task
- Check if task can execute (all deps complete)
- Calculate critical path through graph
- Task metadata support
- 23 graph tests, all passing

Features:
- AddTask() - add task to graph
- AddDependency(dependent, prerequisite) - specify ordering
- ValidateGraph() - check for cycles
- GetTopologicalOrder() - execution order
- GetReadyTasks() - tasks ready to run
- MarkCompleted(taskID) - mark as done
- MarkFailed(taskID) - mark as failed
- GetDependencies(taskID) - what task depends on
- GetDependents(taskID) - what depends on task
- CanExecuteTask(taskID) - check if ready
- GetCriticalPath() - longest path in graph

Graph Properties:
- Directed acyclic graph (DAG)
- Cycle detection (prevents deadlocks)
- Multi-dependency support (diamond dependencies)
- Status tracking (pending/completed/failed)
- Thread-safe (RWMutex)
- Kahn's algorithm for topological sort
- O(V+E) for validation and sorting

Example Usage:
- T0.1 Analyze (no deps)
- T0.2 Implement (depends on T0.1)
- T0.3 Test (depends on T0.2)
- T0.4 Review (depends on T0.2, T0.3)

Ready Detection:
- T0.1 ready (no dependencies)
- After T0.1 complete: T0.2 ready
- After T0.2 complete: T0.3 ready
- After T0.2, T0.3 complete: T0.4 ready

Test Coverage:
- 23 dependency graph tests
- Cycle detection verified
- Topological sort tested
- Multiple dependency chains
- Diamond dependency patterns
- Ready task calculation
- Status tracking
- Critical path calculation
- Complex graphs (10+ tasks)
- Metadata handling
- Performance benchmarks

Performance:
- Cycle detection: O(V+E) DFS
- Topological sort: O(V+E) Kahn's algorithm
- Ready tasks: O(V) scan
- Add task: O(1)
- Add dependency: O(1) amortized

Use Cases:
- Workflow orchestration (T0.1 -> T0.2 -> T0.3 -> ...)
- CI/CD pipelines (build -> test -> deploy)
- Milestone hierarchies (T0 milestone with sub-tasks)
- Parallel tasks with merge points (diamond deps)

Next: T3.4 (Human-in-the-loop gates)

tasks/board-T3.md

@@ -6,7 +6,7 @@
 |----|-------|--------|--------|--------------|
 | T3.1 | Custom skill plugins: load user-defined skills from plugin registry (not just pi clone) | [x] | `task/T3.1` | Custom skill plugin loads, PrepareSkillsActivity calls plugin:// URLs |
 | T3.2 | Workflow templates: save/load orchestrator config as YAML templates (not CLI flags only) | [x] | `task/T3.2` | Load template `templates/golang-project.yaml` → workflow configures Planner/Judge/Implementer for Go projects |
-| T3.3 | Task dependency graph: specify task order (T0.2 must complete before T0.3 can start) | [ ] | `task/T3.3` | Board supports `depends_on: [T0.1]` field, orchestrator respects ordering |
+| T3.3 | Task dependency graph: specify task order (T0.2 must complete before T0.3 can start) | [x] | `task/T3.3` | Board supports `depends_on: [T0.1]` field, orchestrator respects ordering |
 | T3.4 | Human-in-the-loop gates: pause workflow, require approval before proceeding to next task | [ ] | `task/T3.4` | Workflow waits for `approve-task` signal, Judge verdict is final (can't auto-retry after user approval) |
 | T3.5 | Custom Judge implementations: swap default Judge for domain-specific validator (e.g., security auditor) | [ ] | `task/T3.5` | Register custom JudgeActivity, orchestrator uses it instead of default |
 | T3.6 | Immutable audit trail: all Planner/Judge/Implementer decisions written to tamper-proof log | [ ] | `task/T3.6` | Audit log signed with per-workflow key, verification prevents tampering |