feat: add universal document sharding support with dual-strategy loading

Implement comprehensive document sharding system across all BMM workflows enabling 90%+ token savings for large multi-epic projects through selective loading optimization.

## Document Sharding System

### Core Features
- **Universal Support**: All 12 BMM workflows (Phase 1-4) handle both whole and sharded documents
- **Dual Loading Strategy**: Full Load (Phase 1-3) vs Selective Load (Phase 4)
- **Automatic Discovery**: Workflows detect format transparently (whole → sharded priority)
- **Efficiency Optimization**: 90%+ token reduction for 10+ epic projects in Phase 4

### Implementation Details

**Phase 1-3 Workflows (7 workflows) - Full Load Strategy:**
- product-brief, prd, gdd, create-ux-design, tech-spec, architecture, solutioning-gate-check
- Load entire sharded documents when present
- Transparent to user experience
- Better organization for large projects

**Phase 4 Workflows (5 workflows) - Selective Load Strategy:**
- sprint-planning (Full Load exception - needs all epics)
- epic-tech-context, create-story, story-context, code-review (Selective Load)
- Load ONLY the specific epic needed (e.g., epic-3.md for Epic 3 stories)
- Massive efficiency: Skip loading 9 other epics in 10-epic project

### Workflow Enhancements

**Added to all workflows:**
- `input_file_patterns` in workflow.yaml with wildcard discovery
- Document Discovery section in instructions.md
- Support for sharded index + section files
- Brownfield `docs/index.md` support

**Pattern standardization:**
```yaml
input_file_patterns:
  document:
    whole: "{output_folder}/*doc*.md"
    sharded: "{output_folder}/*doc*/index.md"
    sharded_single: "{output_folder}/*doc*/section-{{id}}.md"  # Selective load
```

### Retrospective Workflow Major Overhaul

Transformed retrospective into immersive, interactive team experience:

**Epic Discovery Priority (Fixed):**
- Priority 1: Check sprint-status.yaml for last completed epic
- Priority 2: Ask user directly
- Priority 3: Scan stories folder (last resort)

**New Capabilities:**
- Deep story analysis: Extract dev notes, mistakes, review feedback, lessons learned
- Previous retro integration: Track action items, verify lessons applied
- Significant change detection: Alert when discoveries require epic updates
- Intent-based facilitation: Natural conversation vs scripted phrases
- Party mode protocol: Clear speaker identification (Name (Role): dialogue)
- Team dynamics: Drama, disagreements, diverse perspectives, authentic conflict

**Structure:**
- 12 whole-number steps (no decimals)
- Highly interactive with constant user engagement
- Cross-references previous retro for accountability
- Synthesizes patterns across all stories
- Detects architectural assumption changes

## Documentation

**Created:**
- `docs/document-sharding-guide.md` - Comprehensive 300+ line guide
  - What is sharding, when to use it (token thresholds)
  - How sharding works (discovery system, loading strategies)
  - Using shard-doc tool
  - Full Load vs Selective Load patterns
  - Complete examples and troubleshooting
  - Custom workflow integration patterns

**Updated:**
- `README.md` - Added Document Sharding feature section
- `docs/index.md` - Added under Advanced Topics → Optimization
- `src/modules/bmm/workflows/README.md` - Added sharding section with usage
- `src/modules/bmb/workflows/create-workflow/workflow-creation-guide.md` - Added complete implementation patterns for workflow builders

**Documentation levels:**
1. Overview (README.md) - Quick feature highlight
2. User guide (BMM workflows README) - Practical usage
3. Reference (document-sharding-guide.md) - Complete details
4. Builder guide (workflow-creation-guide.md) - Implementation patterns

## Efficiency Gains

**Example: 10-Epic Project**

Before sharding:
- epic-tech-context for Epic 3: Load all 10 epics (~50k tokens)
- create-story for Epic 3: Load all 10 epics (~50k tokens)
- story-context for Epic 3: Load all 10 epics (~50k tokens)

After sharding with selective load:
- epic-tech-context for Epic 3: Load Epic 3 only (~5k tokens) = 90% reduction
- create-story for Epic 3: Load Epic 3 only (~5k tokens) = 90% reduction
- story-context for Epic 3: Load Epic 3 only (~5k tokens) = 90% reduction

## Breaking Changes

None - fully backward compatible. Workflows work with existing whole documents.

## Files Changed

**Workflows Updated (25 files):**
- 7 Phase 1-3 workflows: Added full load sharding support
- 5 Phase 4 workflows: Added selective load sharding support
- 1 retrospective workflow: Complete overhaul with sharding support

**Documentation (5 files):**
- Created: document-sharding-guide.md
- Updated: README.md, docs/index.md, BMM workflows README, BMB workflow-creation-guide
- Removed: Old conversion report (obsolete)

## Future Extensibility

- BMB workflows now aware of sharding patterns
- Custom modules can easily implement sharding support
- Standard patterns documented for consistency
- No need to explain concept in future development

src/modules/bmm/workflows/4-implementation/code-review/instructions.md

@@ -16,6 +16,35 @@
 
 <critical>DOCUMENT OUTPUT: Technical review reports. Structured findings with severity levels and action items. User skill level ({user_skill_level}) affects conversation style ONLY, not review content.</critical>
 
+## 📚 Document Discovery - Selective Epic Loading
+
+**Strategy**: This workflow needs only ONE specific epic and its stories for review context, not all epics. This provides huge efficiency gains when epics are sharded.
+
+**Epic Discovery Process (SELECTIVE OPTIMIZATION):**
+
+1. **Determine which epic** you need (epic_num from story being reviewed - e.g., story "3-2-feature-name" needs Epic 3)
+2. **Check for sharded version**: Look for `epics/index.md`
+3. **If sharded version found**:
+   - Read `index.md` to understand structure
+   - **Load ONLY `epic-{epic_num}.md`** (e.g., `epics/epic-3.md` for Epic 3)
+   - DO NOT load all epic files - only the one needed!
+   - This is the key efficiency optimization for large multi-epic projects
+4. **If whole document found**: Load the complete `epics.md` file and extract the relevant epic
+
+**Other Documents (architecture, ux-design) - Full Load:**
+
+1. **Search for whole document first** - Use fuzzy file matching
+2. **Check for sharded version** - If whole document not found, look for `{doc-name}/index.md`
+3. **If sharded version found**:
+   - Read `index.md` to understand structure
+   - Read ALL section files listed in the index
+   - Treat combined content as single document
+4. **Brownfield projects**: The `document-project` workflow creates `{output_folder}/docs/index.md`
+
+**Priority**: If both whole and sharded versions exist, use the whole document.
+
+**UX-Heavy Projects**: Always check for ux-design documentation as it provides critical context for reviewing UI-focused stories.
+
 <workflow>
 
   <step n="1" goal="Find story ready for review" tag="sprint-status">