security: switch cookie-import-browser CDP from TCP port to --remote-debugging-pipe

**Context**

The v20 App-Bound Encryption elevation path on Windows, deferred from the v1.6.0.0 security wave. See the updated comment block in `browse/src/cookie-import-browser.ts` around the `--remote-debugging-port` launch.

**Problem**

On Windows 10.15+ with ABE v20, a local same-user process cannot decrypt cookies by reading the SQLite DB directly — the DPAPI context is bound to the browser process. But the CDP debug port bypasses that: any same-user process can connect to `http://127.0.0.1:/json/list`, find a page target's WebSocket URL, and call `Network.getAllCookies` inside Chrome to get decrypted values.

Today the port is randomized in [9222, 9321] and Chrome is killed in the finally block, but there's a window (typically 1-3 seconds while the CDP call is in flight) during which a concurrent same-user process can steal decrypted v20 cookies.

**Fix direction**

Switch `--remote-debugging-port=` → `--remote-debugging-pipe`. Chrome then exposes the CDP transport over a parent/child stdio pipe instead of a TCP socket, eliminating the TCP surface entirely.

**Work**

- Drop `--remote-debugging-port` from the Bun.spawn args in `cookie-import-browser.ts` around line 846.
- Pass `--remote-debugging-pipe` + `stdio: ['pipe', 'pipe', 'pipe', 'pipe', 'pipe']` (CDP uses fds 3+4).
- Replace `fetch(http://127.0.0.1:\${debugPort}/json/list)` + WebSocket connection with a CDP client that reads/writes framed JSON on the pipe fds.
- Playwright doesn't expose this transport out of the box. Either write a minimal CDP-over-pipe client (~200 lines) or find a library that supports it.

**Acceptance**

- `bun test` passes, cookie-import flow still works on macOS and Linux.
- Windows integration smoke test confirms v20 cookies are imported.
- No TCP port opened for the debug transport.

**Out of scope for this issue**

- Restructuring extractCookiesViaCdp for non-Chromium targets.
- Switching the main browse daemon to pipe transport (separate concern).

Tracked from v1.6.0.0 security wave N2 non-goal.