feat: add SelfHealingManager for unattended multi-day operation

Adds four self-healing subsystems to enable the engine to recover from
common failure modes without human intervention:

- Auto-unpause: clears rate-limit-triggered globalPause with escalating
  backoff (5 min → 60 min cap), resets on sustained recovery
- Stuck kill budget: caps task stuck-kill retries (default 3) to prevent
  infinite stuck→todo→stuck loops
- Periodic maintenance (every 15 min): git worktree prune, orphan cleanup,
  SQLite WAL checkpoint
- Worktree cap enforcement: removes oldest idle worktrees when count
  exceeds 2× maxWorktrees

New settings: autoUnpauseEnabled, autoUnpauseBaseDelayMs,
autoUnpauseMaxDelayMs, maxStuckKills, maintenanceIntervalMs.
New task field: stuckKillCount (schema v8 migration).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/core/src/db.ts

@@ -59,7 +59,7 @@ export function fromJson<T>(json: string | null | undefined): T | undefined {
 
 // ── Schema Definition ────────────────────────────────────────────────
 
-const SCHEMA_VERSION = 7;
+const SCHEMA_VERSION = 8;
 
 function normalizeTaskComments(
   steeringComments: SteeringComment[] | undefined,
@@ -418,8 +418,14 @@ export class Database {
       });
     }
 
+    if (version < 8) {
+      this.applyMigration(8, () => {
+        this.addColumnIfMissing("tasks", "stuckKillCount", "INTEGER DEFAULT 0");
+      });
+    }
+
     // Future migrations go here:
-    // if (version < 8) { this.applyMigration(8, () => { ... }); }
+    // if (version < 9) { this.applyMigration(9, () => { ... }); }
   }
 
   /**
@@ -486,6 +492,15 @@ export class Database {
     }
   }
 
+  /**
+   * Run a WAL checkpoint to truncate the WAL file and reclaim disk space.
+   * Safe to call periodically. Returns checkpoint stats.
+   */
+  walCheckpoint(): { busy: number; log: number; checkpointed: number } {
+    const row = this.db.prepare("PRAGMA wal_checkpoint(TRUNCATE)").get() as any;
+    return { busy: row?.busy ?? 0, log: row?.log ?? 0, checkpointed: row?.checkpointed ?? 0 };
+  }
+
   /**
    * Close the database connection.
    */