refactor: remove testing agent claim mechanism for concurrent testing

Remove the testing_in_progress claim/release mechanism from the testing
agent architecture. Multiple testing agents can now test the same feature
concurrently, simplifying the system and eliminating potential stale lock
issues.

Changes:
- parallel_orchestrator.py:
  - Remove claim_feature_for_testing() and release_testing_claim() methods
  - Remove _cleanup_stale_testing_locks() periodic cleanup
  - Replace with simple _get_random_passing_feature() selection
  - Remove startup stale lock cleanup code
  - Remove STALE_TESTING_LOCK_MINUTES constant
  - Remove unused imports (timedelta, text)

- api/database.py:
  - Remove testing_in_progress and last_tested_at columns from Feature model
  - Update to_dict() to exclude these fields
  - Convert _migrate_add_testing_columns() to no-op for backwards compat

- mcp_server/feature_mcp.py:
  - Remove feature_release_testing tool entirely
  - Remove unused datetime import

- prompts.py:
  - Update testing prompt to remove feature_release_testing instruction
  - Testing agents now just verify and exit (no cleanup needed)

- server/websocket.py:
  - Update AgentTracker to use composite keys (feature_id, agent_type)
  - Prevents ghost agent creation from ambiguous [Feature #X] messages
  - Proper separation of coding vs testing agent tracking

Benefits:
- Eliminates artificial bottleneck from claim coordination
- No stale locks to clean up after crashes
- Simpler crash recovery (no testing state to restore)
- Reduced database writes (no claim/release transactions)
- Matches intended design: random, concurrent regression testing

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

api/database.py

@@ -57,10 +57,6 @@ class Feature(Base):
     # Dependencies: list of feature IDs that must be completed before this feature
     # NULL/empty = no dependencies (backwards compatible)
     dependencies = Column(JSON, nullable=True, default=None)
-    # Regression testing: prevent concurrent testing of the same feature
-    testing_in_progress = Column(Boolean, nullable=False, default=False, index=True)
-    # Last time this feature was tested (for session-based regression tracking)
-    last_tested_at = Column(DateTime, nullable=True, default=None)
 
     def to_dict(self) -> dict:
         """Convert feature to dictionary for JSON serialization."""
@@ -76,9 +72,6 @@ class Feature(Base):
             "in_progress": self.in_progress if self.in_progress is not None else False,
             # Dependencies: NULL/empty treated as empty list for backwards compat
             "dependencies": self.dependencies if self.dependencies else [],
-            # Regression testing fields
-            "testing_in_progress": self.testing_in_progress if self.testing_in_progress is not None else False,
-            "last_tested_at": self.last_tested_at.isoformat() if self.last_tested_at else None,
         }
 
     def get_dependencies_safe(self) -> list[int]:
@@ -240,23 +233,18 @@ def _migrate_add_dependencies_column(engine) -> None:
 
 
 def _migrate_add_testing_columns(engine) -> None:
-    """Add testing_in_progress and last_tested_at columns for regression testing.
+    """Legacy migration - no longer adds testing columns.
 
-    These columns support atomic claiming of features for regression testing
-    and tracking when features were last tested in a session.
+    The testing_in_progress and last_tested_at columns were removed from the
+    Feature model as part of simplifying the testing agent architecture.
+    Multiple testing agents can now test the same feature concurrently
+    without coordination.
+
+    This function is kept for backwards compatibility but does nothing.
+    Existing databases with these columns will continue to work - the columns
+    are simply ignored.
     """
-    with engine.connect() as conn:
-        # Check existing columns
-        result = conn.execute(text("PRAGMA table_info(features)"))
-        columns = [row[1] for row in result.fetchall()]
-
-        if "testing_in_progress" not in columns:
-            conn.execute(text("ALTER TABLE features ADD COLUMN testing_in_progress BOOLEAN DEFAULT 0"))
-            conn.commit()
-
-        if "last_tested_at" not in columns:
-            conn.execute(text("ALTER TABLE features ADD COLUMN last_tested_at DATETIME DEFAULT NULL"))
-            conn.commit()
+    pass
 
 
 def _is_network_path(path: Path) -> bool: