feat(FN-709): store branch name on tasks for reliable merge recovery

- Add 'branch' field to Task and ArchivedTaskEntry types with DB migration
- Executor stores branch name on task after worktree assignment
- Merger reads branch from task metadata instead of relying on worktree state
- Implement non-destructive conflict recovery in prepareForTask with reset/clean
- Add tests for branch storage, merger branch reading, and worktree recovery

packages/core/src/db.test.ts

@@ -86,7 +86,7 @@ describe("Database", () => {
     });
 
     it("seeds schema version", () => {
-      expect(db.getSchemaVersion()).toBe(5);
+      expect(db.getSchemaVersion()).toBe(6);
     });
 
     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(5);
+      expect(db.getSchemaVersion()).toBe(6);
     });
 
     it("does not overwrite existing config on re-init", () => {
@@ -704,7 +704,7 @@ 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(5);
+    expect(db.getSchemaVersion()).toBe(6);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
@@ -729,11 +729,11 @@ describe("schema migrations", () => {
     const db = new Database(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(5);
+    expect(db.getSchemaVersion()).toBe(6);
 
     // Re-init should not fail
     db.init();
-    expect(db.getSchemaVersion()).toBe(5);
+    expect(db.getSchemaVersion()).toBe(6);
 
     db.close();
   });
@@ -828,13 +828,14 @@ describe("schema migrations", () => {
     db.init();
 
     // Verify version bumped to 5
-    expect(db.getSchemaVersion()).toBe(5);
+    expect(db.getSchemaVersion()).toBe(6);
 
     // Verify new columns exist and existing data is intact
     const cols = db.prepare("PRAGMA table_info(tasks)").all() as Array<{ name: string }>;
     const colNames = cols.map((c) => c.name);
     expect(colNames).toContain("missionId");
     expect(colNames).toContain("sliceId");
+    expect(colNames).toContain("branch");
 
     // Existing task should still be readable
     const task = db.prepare("SELECT * FROM tasks WHERE id = 'KB-2'").get() as any;
@@ -1037,7 +1038,7 @@ describe("createDatabase factory", () => {
     const db = createDatabase(kbDir);
     db.init();
 
-    expect(db.getSchemaVersion()).toBe(5);
+    expect(db.getSchemaVersion()).toBe(6);
     expect(db.getLastModified()).toBeGreaterThan(0);
 
     db.close();