Add in-progress status tracking for features

Implements feature locking to prevent multiple agent sessions from working
on the same feature simultaneously. This is essential for parallel agent
execution.

Database changes:
- Add `in_progress` boolean column to Feature model
- Add migration function to handle existing databases

MCP Server tools:
- Add `feature_mark_in_progress` - lock feature when starting work
- Add `feature_clear_in_progress` - unlock feature when abandoning
- Update `feature_get_next` to skip in-progress features
- Update `feature_get_stats` to include in_progress count
- Update `feature_mark_passing` and `feature_skip` to clear in_progress

Backend updates:
- Update progress.py to track and display in_progress count
- Update features router to properly categorize in-progress features
- Update WebSocket to broadcast in_progress in progress updates
- Add in_progress to FeatureResponse schema

Frontend updates:
- Add in_progress to TypeScript types (Feature, ProjectStats, WSProgressMessage)
- Update useWebSocket hook to track in_progress state

Prompt template:
- Add instructions for agents to mark features in-progress immediately
- Document new MCP tools in allowed tools section

Also fixes spec_chat_session.py to use absolute project path instead of
relative path for consistency with CLI behavior.

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

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

ui/src/lib/types.ts

@@ -5,6 +5,7 @@
 // Project types
 export interface ProjectStats {
   passing: number
+  in_progress: number
   total: number
   percentage: number
 }
@@ -34,6 +35,7 @@ export interface Feature {
   description: string
   steps: string[]
   passes: boolean
+  in_progress: boolean
 }
 
 export interface FeatureListResponse {
@@ -79,6 +81,7 @@ export type WSMessageType = 'progress' | 'feature_update' | 'log' | 'agent_statu
 export interface WSProgressMessage {
   type: 'progress'
   passing: number
+  in_progress: number
   total: number
   percentage: number
 }