v10.5.0: Termux/Android support + HTTP-only graceful degradation

Termux/Android install fix + HTTP-only graceful degradation

The lean install (pip install hound-mcp) no longer pulls playwright (which has no wheels for Termux/aarch64). Browser deps moved to the [all] extra with loose pins.

When browser deps are unavailable, hound runs in HTTP-only mode: fetch, search, and crawl work via httpx + trafilatura. Stealthy browser escalation and screenshot are disabled with clear error messages.

What works in HTTP-only mode: web_fetch (HTTP tier), web_search, web_crawl, cache_clear, hound_version
Disabled: stealthy browser escalation, web_screenshot, persistent browser sessions

Doctor adds a browser deps check (non-blocking, reports HTTP-only mode when missing).

742 tests pass (18 new graceful degradation tests).