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/status.md

@@ -108,7 +108,7 @@ ${sortedFeatures.map(f => `
 
 ### Quick Access
 - View session details: /workflow:status
-- Archive query: jq '.archives[] | select(.session_id == "SESSION_ID")' .workflow/.archives/manifest.json
+- Archive query: jq '.archives[] | select(.session_id == "SESSION_ID")' .workflow/archives/manifest.json
 - Documentation: .workflow/docs/${projectData.project_name}/
 
 ### Query Commands
@@ -139,7 +139,7 @@ Complete your first workflow session to add features:
 
 ```bash
 # List 5 most recent archived sessions
-bash(ls -1t .workflow/.archives/WFS-* 2>/dev/null | head -5 | xargs -I {} basename {})
+bash(ls -1t .workflow/archives/WFS-* 2>/dev/null | head -5 | xargs -I {} basename {})
 ```
 
 **Output**:
@@ -156,28 +156,28 @@ Use /workflow:session:complete to archive current session.
 
 ### Step 1: Find Active Session
 ```bash
-find .workflow/ -name ".active-*" -type f 2>/dev/null | head -1
+find .workflow/sessions/ -name "WFS-*" -type d 2>/dev/null | head -1
 ```
 
 ### Step 2: Load Session Data
 ```bash
-cat .workflow/WFS-session/workflow-session.json
+cat .workflow/sessions/WFS-session/workflow-session.json
 ```
 
 ### Step 3: Scan Task Files
 ```bash
-find .workflow/WFS-session/.task/ -name "*.json" -type f 2>/dev/null
+find .workflow/sessions/WFS-session/.task/ -name "*.json" -type f 2>/dev/null
 ```
 
 ### Step 4: Generate Task Status
 ```bash
-cat .workflow/WFS-session/.task/impl-1.json | jq -r '.status'
+cat .workflow/sessions/WFS-session/.task/impl-1.json | jq -r '.status'
 ```
 
 ### Step 5: Count Task Progress
 ```bash
-find .workflow/WFS-session/.task/ -name "*.json" -type f | wc -l
-find .workflow/WFS-session/.summaries/ -name "*.md" -type f 2>/dev/null | wc -l
+find .workflow/sessions/WFS-session/.task/ -name "*.json" -type f | wc -l
+find .workflow/sessions/WFS-session/.summaries/ -name "*.md" -type f 2>/dev/null | wc -l
 ```
 
 ### Step 6: Display Overview