Make KB Capture project-centric

- Sessions now live inside project folders: <Project>/_capture/<session>/
- Project picker dropdown (scans projects folder)
- Auto-discovers projects with KB/ folder or _context.md
- Windows: D:/ATODrive/Projects
- Linux: ~/obsidian-vault/2-Projects

This aligns with the KB structure where Mario updates:
- KB/dev/gen-XXX.md (session captures)
- Images/screenshot-sessions/ (frames)

src/cad_documenter/session.py

@@ -116,27 +116,61 @@ class SessionManager:
     """
     Manages recording sessions and clips.
     
-    Directory structure:
-        sessions/
-        ├── <session-id>/
-        │   ├── session.json       # Session metadata
-        │   ├── clips/
-        │   │   ├── clip-001.mp4
-        │   │   ├── clip-002.mp4
-        │   │   └── ...
-        │   └── export/            # Final export for Clawdbot
-        │       ├── merged.mp4
-        │       ├── transcript.json
-        │       └── metadata.json
+    Project-centric structure:
+        /2-Projects/<ProjectName>/
+        ├── KB/
+        │   └── dev/                    # gen-XXX.md session captures (Mario creates)
+        ├── Images/
+        │   └── screenshot-sessions/    # Frames organized by session
+        └── _capture/                   # Session staging
+            └── <session-id>/
+                ├── session.json
+                ├── clips/
+                │   ├── clip-001.mp4
+                │   └── ...
+                └── clawdbot_export/    # Ready for Mario
+                    ├── merged.mp4
+                    ├── transcript.json
+                    └── metadata.json
     """
     
-    def __init__(self, base_path: Path):
-        self.base_path = Path(base_path)
-        self.sessions_dir = self.base_path / "sessions"
-        self.sessions_dir.mkdir(parents=True, exist_ok=True)
+    def __init__(self, projects_root: Path):
+        """
+        Initialize session manager.
         
+        Args:
+            projects_root: Path to projects folder (e.g., /2-Projects/ or D:/ATODrive/Projects/)
+        """
+        self.projects_root = Path(projects_root)
         self.current_session: Optional[Session] = None
         self.current_clip: Optional[Clip] = None
+        self._current_project_path: Optional[Path] = None
+    
+    def list_projects(self) -> List[str]:
+        """List available projects (folders in projects_root)."""
+        projects = []
+        if self.projects_root.exists():
+            for p in sorted(self.projects_root.iterdir()):
+                if p.is_dir() and not p.name.startswith((".", "_")):
+                    # Check if it looks like a project (has KB folder or _context.md)
+                    if (p / "KB").exists() or (p / "_context.md").exists():
+                        projects.append(p.name)
+        return projects
+    
+    def get_project_path(self, project: str) -> Path:
+        """Get full path to a project."""
+        return self.projects_root / project
+    
+    def get_capture_dir(self, project: str) -> Path:
+        """Get the _capture directory for a project."""
+        return self.get_project_path(project) / "_capture"
+    
+    @property
+    def sessions_dir(self) -> Path:
+        """Current project's capture directory."""
+        if self._current_project_path:
+            return self._current_project_path / "_capture"
+        raise RuntimeError("No project selected")
     
     def start_session(
         self,
@@ -144,7 +178,12 @@ class SessionManager:
         project: str,
         session_type: SessionType = SessionType.DESIGN,
     ) -> Session:
-        """Start a new recording session."""
+        """Start a new recording session within a project."""
+        # Set current project
+        self._current_project_path = self.get_project_path(project)
+        if not self._current_project_path.exists():
+            raise ValueError(f"Project not found: {project}")
+        
         session_id = datetime.now().strftime("%Y%m%d-%H%M%S")
         
         session = Session(
@@ -155,8 +194,9 @@ class SessionManager:
             created_at=datetime.now(),
         )
         
-        # Create session directory
-        session_dir = self.sessions_dir / session_id
+        # Create session directory in project's _capture folder
+        capture_dir = self._current_project_path / "_capture"
+        session_dir = capture_dir / session_id
         session_dir.mkdir(parents=True, exist_ok=True)
         (session_dir / "clips").mkdir(exist_ok=True)
         
@@ -299,15 +339,26 @@ class SessionManager:
                 return Session.from_dict(json.load(f))
         return None
     
-    def list_sessions(self) -> List[Session]:
-        """List all sessions."""
+    def list_sessions(self, project: Optional[str] = None) -> List[Session]:
+        """List sessions, optionally filtered by project."""
         sessions = []
-        for session_dir in sorted(self.sessions_dir.iterdir(), reverse=True):
-            if session_dir.is_dir():
-                session_file = session_dir / "session.json"
-                if session_file.exists():
-                    with open(session_file) as f:
-                        sessions.append(Session.from_dict(json.load(f)))
+        
+        if project:
+            # List sessions for specific project
+            capture_dir = self.get_capture_dir(project)
+            if capture_dir.exists():
+                for session_dir in sorted(capture_dir.iterdir(), reverse=True):
+                    if session_dir.is_dir():
+                        session_file = session_dir / "session.json"
+                        if session_file.exists():
+                            with open(session_file) as f:
+                                sessions.append(Session.from_dict(json.load(f)))
+        else:
+            # List sessions across all projects
+            for proj in self.list_projects():
+                sessions.extend(self.list_sessions(proj))
+            sessions.sort(key=lambda s: s.created_at, reverse=True)
+        
         return sessions
     
     def get_session_dir(self, session_id: str) -> Path: