feat: Add arbitrary directory project storage with registry system

This major update replaces the fixed `generations/` directory with support
for storing projects in any directory on the filesystem. Projects are now
tracked via a cross-platform registry system.

## New Features

### Project Registry (`registry.py`)
- Cross-platform registry storing project name-to-path mappings
- Platform-specific config locations:
  - Windows: %APPDATA%\autonomous-coder\projects.json
  - macOS: ~/Library/Application Support/autonomous-coder/projects.json
  - Linux: ~/.config/autonomous-coder/projects.json
- POSIX path format for cross-platform compatibility
- File locking for concurrent access safety (fcntl/msvcrt)
- Atomic writes via temp file + rename to prevent corruption
- Fixed Windows file locking issue with tempfile.mkstemp()

### Filesystem Browser API (`server/routers/filesystem.py`)
- REST endpoints for browsing directories server-side
- Cross-platform support with blocked system paths:
  - Windows: C:\Windows, Program Files, ProgramData, etc.
  - macOS: /System, /Library, /private, etc.
  - Linux: /etc, /var, /usr, /bin, etc.
- Universal blocked paths: .ssh, .aws, .gnupg, .docker, etc.
- Hidden file detection (Unix dot-prefix + Windows attributes)
- UNC path blocking for security
- Windows drive enumeration via ctypes
- Directory creation with validation
- Added `has_children` field to DirectoryEntry schema

### UI Folder Browser (`ui/src/components/FolderBrowser.tsx`)
- React component for selecting project directories
- Breadcrumb navigation with clickable segments
- Windows drive selector
- New folder creation inline
- Fixed text visibility with explicit color values

## Updated Components

### Server Routers
- `projects.py`: Uses registry instead of fixed generations/ directory
- `agent.py`: Uses registry for project path lookups
- `features.py`: Uses registry for database path resolution
- `spec_creation.py`: Uses registry for WebSocket project resolution

### Process Manager (`server/services/process_manager.py`)
- Fixed sandbox issue: subprocess now uses project_dir as cwd
- This allows the Claude SDK sandbox to access external project directories

### Schemas (`server/schemas.py`)
- Added `has_children` to DirectoryEntry
- Added `in_progress` to ProjectStats
- Added path field to ProjectSummary and ProjectDetail

### UI Components
- `NewProjectModal.tsx`: Multi-step wizard with folder selection
- Added clarifying text about subfolder creation
- Fixed text color visibility issues

### API Client (`ui/src/lib/api.ts`)
- Added filesystem API functions (listDirectory, createDirectory)
- Fixed Windows path splitting for directory creation

### Documentation
- Updated CLAUDE.md with registry system details
- Updated command examples for absolute paths

## Security Improvements
- Blocked `.` and `..` in directory names to prevent traversal
- Added path blocking check in project creation
- UNC path blocking throughout filesystem API

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/services/spec_chat_session.py

@@ -35,15 +35,16 @@ class SpecChatSession:
     - Phase 6-7: Success Criteria & Approval
     """
 
-    def __init__(self, project_name: str):
+    def __init__(self, project_name: str, project_dir: Path):
         """
         Initialize the session.
 
         Args:
             project_name: Name of the project being created
+            project_dir: Absolute path to the project directory
         """
         self.project_name = project_name
-        self.project_dir = ROOT_DIR / "generations" / project_name
+        self.project_dir = project_dir
         self.client: Optional[ClaudeSDKClient] = None
         self.messages: list[dict] = []
         self.complete: bool = False
@@ -342,14 +343,19 @@ def get_session(project_name: str) -> Optional[SpecChatSession]:
         return _sessions.get(project_name)
 
 
-async def create_session(project_name: str) -> SpecChatSession:
-    """Create a new session for a project, closing any existing one."""
+async def create_session(project_name: str, project_dir: Path) -> SpecChatSession:
+    """Create a new session for a project, closing any existing one.
+
+    Args:
+        project_name: Name of the project
+        project_dir: Absolute path to the project directory
+    """
     old_session: Optional[SpecChatSession] = None
 
     with _sessions_lock:
         # Get existing session to close later (outside the lock)
         old_session = _sessions.pop(project_name, None)
-        session = SpecChatSession(project_name)
+        session = SpecChatSession(project_name, project_dir)
         _sessions[project_name] = session
 
     # Close old session outside the lock to avoid blocking