feat: Redesign test-gen workflow with independent session and cross-session context

Major redesign of test-gen workflow based on simplified architecture:

Core Changes:
- Creates independent test session (WFS-test-[source]) instead of modifying existing session
- Implements 4-phase execution with concept-enhanced analysis integration
- Reuses IMPL-*.json format with meta.type="test-fix" (no new task type)
- Auto-detects cross-session context via workflow-session.json metadata

4-Phase Workflow:
1. Phase 1: Create test session with workflow_type and source_session_id metadata
2. Phase 2: Auto-detect and gather cross-session context (summaries, git changes, tests)
3. Phase 3: Analyze implementation with concept-enhanced (parallel Gemini + Codex)
4. Phase 4: Generate IMPL-001.json task with test strategy from ANALYSIS_RESULTS.md

Key Features:
- Session isolation: Test session separate from implementation session
- Parameter simplification: context-gather auto-detects via metadata (no --source-session param)
- Format reuse: IMPL-*.json with meta.type field (zero changes to execute/status tools)
- Analysis integration: concept-enhanced provides comprehensive test strategy and code targets
- Automatic cross-session: Collects source session artifacts automatically

Tool Modifications Required:
- session-start: Support test session metadata (workflow_type, source_session_id)
- context-gather: Auto-detect test session and gather from source session
- task-generate: Extract test strategy from ANALYSIS_RESULTS.md
- concept-enhanced: No changes (already supports analysis)
- execute: No changes (already dispatches by meta.agent)

Data Flow:
User → session-start (creates test session) → context-gather (auto cross-session)
→ concept-enhanced (analyzes + generates strategy) → task-generate (creates task)
→ Returns summary → User triggers /workflow:execute separately

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: catlog22