fix(twitter): detect degraded search health

[Huanshere]

What changed

• Detect Failed to init ClientTransaction in twitter status and report twitter-cli as degraded instead of claiming search is fully available.
• Let the existing multi-backend selection continue to a healthy OpenCLI fallback when Twitter search is degraded.
• Update the Twitter retry chain for both uv/pipx installs and add a signed-in browser search fallback.
• Add a regression test for the false-positive doctor result.

Root cause

twitter status can return ok: true while also warning that ClientTransaction initialization failed. X currently requires the resulting x-client-transaction-id for SearchTimeline, so twitter search returns HTTP 404 even though feed, user, and tweet reads still work. The upstream twitter-cli fix is tracked in public-clis/twitter-cli#71.

Validation

• uv run pytest tests/ -q — 256 passed
• uv run pytest tests/test_twitter_channel.py -q — 11 passed
• uv run ruff check agent_reach/channels/twitter.py tests/test_twitter_channel.py — passed
• Installed twitter-cli PR docs: add one-line update guide #71 locally as 0.8.6 and verified twitter search "OpenAI" -n 3 --json returns live results
• uv run python -m agent_reach.cli doctor --json reports Twitter search healthy with the fixed CLI