fix(daemon): detect windowless Chrome before handshake - #659

What

• Probe the local DevTools /json/list endpoint before starting the browser WebSocket handshake.
• Fail fast with a chrome-windowless diagnostic when Chrome consistently exposes no page target.
• Preserve the existing handshake path whenever the probe is unavailable, malformed, non-local, or a page appears during startup.

Why

A local Chrome launched with --no-startup-window cannot render the remote-debugging permission popup. The daemon currently waits for the full 45-second handshake timeout and then tells the user to click a popup that does not exist.

Design

The probe is restricted to loopback ws:// endpoints in local-browser mode. It requires three consecutive empty target lists, spaced 200 ms apart, so a newly launched Chrome can publish its first page target before being classified as windowless. Any ambiguous probe result fails open to the existing handshake behavior.

Verification

• uv run --python 3.12 --with pytest python -m pytest tests/unit/test_daemon.py -q — 38 passed
• Full unit suite on Windows — 166 passed; the same 3 pre-existing platform-specific failures seen on the unmodified baseline remain
• uv run --python 3.12 --with ruff ruff check --select E9,F63,F7,F82 src/browser_harness/daemon.py tests/unit/test_daemon.py
• uv run --python 3.12 --with build python -m build
• Red/green and mutation checks cover no-page detection and the startup race

Fixes #554

Summary by cubic

Fixes the 45-second handshake timeout caused by Chrome launched with --no-startup-window, which cannot show the remote-debugging permission popup. The daemon now probes the local DevTools /json/list endpoint before the handshake and fails fast with a chrome-windowless diagnostic when Chrome consistently exposes no page target.

• The probe only runs on loopback ws:// endpoints and falls back to the existing handshake for any ambiguous or malformed result.
• Requires three consecutive probe responses with no page target, 200 ms apart, so a freshly launched Chrome can publish its first page.

Fixes #554.

^{Written for commit f95d474. Summary will update on new commits.}