fix: code review issues (P1-P3)

- Cookie file caching with mtime check (no disk read per request)
- Streaming retry logic (same as non-streaming)
- Reusable httpx.Client and ssl context (no per-request allocation)
- Shared _extract_texts_from_line() eliminates parsing duplication
- _start_sse() helper eliminates 4x SSE header repetition
- _usage() helper for token estimation
- _parse_body() with proper 400 on invalid JSON
- ValueError guard on @think= parameter
- Single-pass regex in parse_tool_calls
- Fixed __main__.py httpx detection (was always showing 'fallback')
- Removed unused base64 import from gemini.py