feat: decouple regression testing agents from coding agents

Major refactoring of the parallel orchestrator to run regression testing
agents independently from coding agents. This improves system reliability
and provides better control over testing behavior.

Key changes:

Database & MCP Layer:
- Add testing_in_progress and last_tested_at columns to Feature model
- Add feature_claim_for_testing() for atomic test claim with retry
- Add feature_release_testing() to release claims after testing
- Refactor claim functions to iterative loops (no recursion)
- Add OperationalError retry handling for transient DB errors
- Reduce MAX_CLAIM_RETRIES from 10 to 5

Orchestrator:
- Decouple testing agent lifecycle from coding agents
- Add _maintain_testing_agents() for continuous testing maintenance
- Fix TOCTOU race in _spawn_testing_agent() - hold lock during spawn
- Add _cleanup_stale_testing_locks() with 30-min timeout
- Fix log ordering - start_session() before stale flag cleanup
- Add stale testing_in_progress cleanup on startup

Dead Code Removal:
- Remove count_testing_in_concurrency from entire stack (12+ files)
- Remove ineffective with_for_update() from features router

API & UI:
- Pass testing_agent_ratio via CLI to orchestrator
- Update testing prompt template to use new claim/release tools
- Rename UI label to "Regression Agents" with clearer description
- Add process_utils.py for cross-platform process tree management

Testing agents now:
- Run continuously as long as passing features exist
- Can re-test features multiple times to catch regressions
- Are controlled by fixed count (0-3) via testing_agent_ratio setting
- Have atomic claiming to prevent concurrent testing of same feature

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

api/database.py

@@ -50,6 +50,10 @@ 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."""
@@ -65,6 +69,9 @@ 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]:
@@ -225,6 +232,26 @@ def _migrate_add_dependencies_column(engine) -> None:
             conn.commit()
 
 
+def _migrate_add_testing_columns(engine) -> None:
+    """Add testing_in_progress and last_tested_at columns for regression testing.
+
+    These columns support atomic claiming of features for regression testing
+    and tracking when features were last tested in a session.
+    """
+    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()
+
+
 def _is_network_path(path: Path) -> bool:
     """Detect if path is on a network filesystem.
 
@@ -341,6 +368,7 @@ def create_database(project_dir: Path) -> tuple:
     _migrate_add_in_progress_column(engine)
     _migrate_fix_null_boolean_fields(engine)
     _migrate_add_dependencies_column(engine)
+    _migrate_add_testing_columns(engine)
 
     # Migrate to add schedules tables
     _migrate_add_schedules_tables(engine)