feat(vscode): switch the extension to the v2 engine with a rollback switch (#2916)

The extension now runs on the agent-core-v2 engine by default. The
interface, sessions, and workflows do not change. Two rollback paths
exist, and one function makes the decision
(config/vscode-settings.ts):

- the kimi.useAgentCoreV1 setting (temporary; a window reload applies
  the change);
- the KIMI_CODE_LEGACY_FLAG environment variable, which wins over the
  setting and has the same semantics as in the CLI.

An engine startup failure shows an explicit error that names the
rollback setting. There is no silent fallback. CI runs the extension
test suite on both engines: the sharded run covers the default v2
engine, and a new test-vscode-legacy job reruns the suite with
KIMI_CODE_LEGACY_FLAG=1.

To keep the v2 path identical to v1 for every method the extension
uses, this change also completes the v2-backed SDK client and the v2
engine:

- Implement session deletion in the v2 SDK client.
- Implement fork truncation at a turn index in the v2 engine, with the
  same rules as v1, and reject a fork while the source session has an
  active turn.
- Stop the session-level /init run when the turn is cancelled, as v1
  does.
- Read session metadata without the archived field as not-archived, so
  sessions written by the v1 engine open correctly.

The SDK parity suite now covers session deletion, cancel, and fork
truncation. The known-difference list for the methods the extension
uses is empty.

.github/workflows/ci.yml

@@ -66,6 +66,27 @@ jobs:
       - run: pnpm install --frozen-lockfile
       - run: pnpm --filter @moonshot-ai/pi-tui test
 
+  # The VS Code extension suite runs on the default (v2) engine as part of the
+  # sharded root run above; this job reruns it on the legacy v1 engine, which
+  # the extension selects through the rollback env var.
+  test-vscode-legacy:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v6
+
+      - uses: actions/setup-node@v6
+        with:
+          node-version-file: .nvmrc
+          cache: pnpm
+
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm --filter kimi-code test
+        env:
+          KIMI_CODE_LEGACY_FLAG: "1"
+
   test-windows:
     runs-on: windows-latest
     # Temporarily disabled while Windows tests are being stabilized.