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.test.ts

@@ -86,7 +86,7 @@ describe("Database", () => {
     });
 
     it("seeds schema version", () => {
-      expect(db.getSchemaVersion()).toBe(7);
+      expect(db.getSchemaVersion()).toBe(8);
     });
 
     it("seeds lastModified", () => {
@@ -109,7 +109,7 @@ describe("Database", () => {
 
     it("is idempotent - calling init() twice does not fail", () => {
       expect(() => db.init()).not.toThrow();
-      expect(db.getSchemaVersion()).toBe(7);
+      expect(db.getSchemaVersion()).toBe(8);
     });
 
     it("does not overwrite existing config on re-init", () => {
@@ -188,6 +188,18 @@ describe("Database", () => {
     });
   });
 
+  describe("walCheckpoint", () => {
+    it("runs WAL checkpoint and returns stats", () => {
+      const result = db.walCheckpoint();
+      expect(result).toHaveProperty("busy");
+      expect(result).toHaveProperty("log");
+      expect(result).toHaveProperty("checkpointed");
+      expect(typeof result.busy).toBe("number");
+      expect(typeof result.log).toBe("number");
+      expect(typeof result.checkpointed).toBe("number");
+    });
+  });
+
   describe("transactions", () => {
     it("commits on success", () => {
       db.transaction(() => {
@@ -704,7 +716,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5 (includes v1→v2, v2→v3, v3→v4, and v4→v5 migrations)
-    expect(db.getSchemaVersion()).toBe(7);
+    expect(db.getSchemaVersion()).toBe(8);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -729,11 +741,11 @@ describe("schema migrations", () => {
     const db = new Database(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(7);
+    expect(db.getSchemaVersion()).toBe(8);
 
     // Re-init should not fail
     db.init();
-    expect(db.getSchemaVersion()).toBe(7);
+    expect(db.getSchemaVersion()).toBe(8);
 
     db.close();
   });
@@ -828,7 +840,7 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5
-    expect(db.getSchemaVersion()).toBe(7);
+    expect(db.getSchemaVersion()).toBe(8);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -1038,7 +1050,7 @@ describe("createDatabase factory", () => {
     const db = createDatabase(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(7);
+    expect(db.getSchemaVersion()).toBe(8);
     expect(db.getLastModified()).toBeGreaterThan(0);
 
     db.close();