refactor(workflow): migrate to sessions/ subdirectory structure

Refactor workflow directory structure to improve clarity and robustness by introducing dedicated sessions/ subdirectory for active sessions.

## Directory Structure Changes

### Before (Old Structure)
```
.workflow/
├── .active-WFS-session-1    # Marker files (fragile)
├── WFS-session-1/           # Active sessions mixed with config
├── WFS-session-2/
├── project.json
└── .archives/               # Archived sessions
```

### After (New Structure)
```
.workflow/
├── project.json             # Project metadata
├── sessions/                # [NEW] All active sessions
│   └── WFS-session-1/
└── archives/                # Archived sessions (removed dot prefix)
```

## Key Improvements

1. **Physical Isolation**: Active sessions now in dedicated sessions/ subdirectory
2. **No Marker Files**: Removed fragile .active-* marker file mechanism
3. **Directory-Based State**: Session state determined by directory location
   - In sessions/ = active
   - In archives/ = archived
4. **Simpler Discovery**: `ls .workflow/sessions/` instead of `find .workflow/ -name ".active-*"`
5. **Cleaner Root**: .workflow/ root now only contains project.json, sessions/, archives/

## Path Transformations

| Old Pattern | New Pattern |
|-------------|-------------|
| `find .workflow/ -name ".active-*"` | `find .workflow/sessions/ -name "WFS-*" -type d` |
| `.workflow/WFS-[slug]/` | `.workflow/sessions/WFS-[slug]/` |
| `.workflow/.archives/` | `.workflow/archives/` |
| `touch .workflow/.active-*` | *Removed* |
| `rm .workflow/.active-*` | *Removed* |

## Modified Commands

### session/start.md
- Update session creation path to .workflow/sessions/WFS-*/
- Remove .active-* marker file creation
- Update session discovery to use directory listing
- Updated all 3 modes: Discovery, Auto, Force New

### session/complete.md
- Update session move from sessions/ to archives/
- Remove .active-* marker file deletion
- Update manifest path to archives/manifest.json
- Update agent prompts with new paths

### status.md
- Update session discovery to find .workflow/sessions/
- Update all session file path references
- Update archive query examples

### execute.md
- Update session discovery logic
- Update all task file paths to include sessions/ prefix
- Update context package paths
- Update error handling documentation

## Benefits

- **Robustness**: Eliminates marker file state inconsistency risk
- **Clarity**: Clear separation between active and archived sessions
- **Simplicity**: Session discovery is straightforward directory listing
- **Alignment**: Closer to OpenSpec's three-layer structure (specs/changes/archive)

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

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

.claude/commands/workflow/session/complete.md

@@ -25,23 +25,23 @@ Mark the currently active workflow session as complete, analyze it for lessons l
 
 #### Step 1.1: Find Active Session and Get Name
 ```bash
-# Find active marker
-bash(find .workflow/ -name ".active-*" -type f | head -1)
+# Find active session directory
+bash(find .workflow/sessions/ -name "WFS-*" -type d | head -1)
 
-# Extract session name from marker path
-bash(basename .workflow/.active-WFS-session-name | sed 's/^\.active-//')
+# Extract session name from directory path
+bash(basename .workflow/sessions/WFS-session-name)
 ```
 **Output**: Session name `WFS-session-name`
 
 #### Step 1.2: Move Session to Archive
 ```bash
 # Create archive directory if needed
-bash(mkdir -p .workflow/.archives/)
+bash(mkdir -p .workflow/archives/)
 
 # Move session to archive location
-bash(mv .workflow/WFS-session-name .workflow/.archives/WFS-session-name)
+bash(mv .workflow/sessions/WFS-session-name .workflow/archives/WFS-session-name)
 ```
-**Result**: Session now at `.workflow/.archives/WFS-session-name/`
+**Result**: Session now at `.workflow/archives/WFS-session-name/`
 
 ### Phase 2: Agent-Orchestrated Completion (All Data Processing)
 
@@ -60,8 +60,7 @@ Task(
 Complete workflow session archival. Session already moved to archive location.
 
 ## Context
-- Session: .workflow/.archives/WFS-session-name/
-- Active marker: .workflow/.active-WFS-session-name
+- Session: .workflow/archives/WFS-session-name/
 
 ## Tasks
 
@@ -77,15 +76,12 @@ Complete workflow session archival. Session already moved to archive location.
    - Calculate: duration_hours, success_rate, tags (3-5 keywords)
    - Construct complete JSON with session_id, description, archived_at, archive_path, metrics, tags, lessons
 
-5. **Update manifest**: Initialize .workflow/.archives/manifest.json if needed, append entry
+5. **Update manifest**: Initialize .workflow/archives/manifest.json if needed, append entry
 
-6. **Remove active marker**
-
-7. **Return result**: {"status": "success", "session_id": "...", "archived_at": "...", "metrics": {...}, "lessons_summary": {...}}
+6. **Return result**: {"status": "success", "session_id": "...", "archived_at": "...", "metrics": {...}, "lessons_summary": {...}}
 
 ## Error Handling
 - On failure: return {"status": "error", "task": "...", "message": "..."}
-- Do NOT remove marker if failed
   `
 )
 ```
@@ -111,7 +107,6 @@ Complete workflow session archival. Session already moved to archive location.
 - Generate lessons learned analysis
 - Build complete archive metadata
 - Update manifest
-- Remove active marker
 - Return success/error result
 
 ### Phase 3: Update Project Feature Registry
@@ -134,7 +129,7 @@ WARNING: No project.json found. Run /workflow:session:start to initialize.
 
 ```bash
 # Read archived IMPL_PLAN.md
-bash(cat .workflow/.archives/WFS-session-name/IMPL_PLAN.md | head -20)
+bash(cat .workflow/archives/WFS-session-name/IMPL_PLAN.md | head -20)
 ```
 
 **Data Processing** (No agent needed):
@@ -192,7 +187,7 @@ const newFeature = {
   },
   traceability: {
     session_id: sessionId,
-    archive_path: archivePath, // e.g., ".workflow/.archives/WFS-auth-system"
+    archive_path: archivePath, // e.g., ".workflow/archives/WFS-auth-system"
     commit_hash: getLatestCommitHash() || "" // Optional: git rev-parse HEAD
   },
   docs: [],      // Placeholder for future doc links