feat(P01): add ideation engine + ciagent ideate command — IDEATE-01,02,03,17 + MULTI-01

---ci---
phase: 1
milestone: v0.10
status: execute
decisions:
  - id: D-080
    decision: Three-tier ideation (mechanical, backend-enriched, cross-project)
    rationale: Mechanical tier always produces output without backend
    confidence: 0.92
  - id: D-089
    decision: No separate codebase map command
    rationale: Git-native + .ciagent/ covers mapping; avoids tree-sitter dep
    confidence: 0.88
requirements:
  covered:
    - IDEATE-01
    - IDEATE-02
    - IDEATE-03
    - IDEATE-17
    - MULTI-01
---/ci---

Add IdeationEngine core module with 15 signal collectors:
- Uncovered/partial requirements from REQUIREMENTS.md
- Coverage gaps (documented but unimplemented agents)
- Repeated lessons from git history
- Low-confidence decisions from ---ci--- blocks
- Escalation patterns from git history
- Compound solution patterns
- Architecture drift (ARCHITECTURE.md vs src/)
- Verification inversion (missing test files)
- Improvement patterns (cross-referencing lessons + requirements)
- Spec ambiguity (should/could/might patterns)
- Spec missing (common requirement categories)
- Cascade impact (--affected from git diff)
- External signals (npm audit, dependency staleness)
- Cross-project lesson mining

Add ciagent ideate CLI command with flags:
--category, --affected, --spec, --external, --cross-project, --output

Add active_projects to CIAgentConfig (backwards compatible with active_project).
Add IDEATE pipeline stage between RESEARCH and PLAN.
Update IdeationAgent to delegate to IdeationEngine.

533 tests passing.

src/types/pipeline.ts

@@ -4,6 +4,7 @@ export type PipelineStage =
   | "specify"
   | "clarify"
   | "research"
+  | "ideate"
   | "plan"
   | "execute"
   | "test"
@@ -18,6 +19,7 @@ export interface PipelineState {
   specification_loaded: boolean;
   clarify_completed: boolean;
   research_completed: boolean;
+  ideate_completed: boolean;
   plan_completed: boolean;
   execute_completed: boolean;
   test_completed: boolean;
@@ -61,6 +63,7 @@ export const STAGE_ORDER: PipelineStage[] = [
   "specify",
   "clarify",
   "research",
+  "ideate",
   "plan",
   "execute",
   "test",
@@ -85,6 +88,7 @@ export function createInitialPipelineState(
     specification_loaded: false,
     clarify_completed: false,
     research_completed: false,
+    ideate_completed: false,
     plan_completed: false,
     execute_completed: false,
     test_completed: false,