fix(llm): make rerank context size configurable via QMD_RERANK_CONTEXT_SIZE

[mvanhorn]

Fixes #291

Summary

Makes RERANK_CONTEXT_SIZE configurable via the QMD_RERANK_CONTEXT_SIZE environment variable, defaulting to 2048 for backwards compatibility. This follows the existing pattern used by QMD_EXPAND_CONTEXT_SIZE.

CJK content and long query expansions can exceed the 2048 default - the truncation logic (added previously) prevents crashes but aggressively truncates content. Users can now set QMD_RERANK_CONTEXT_SIZE=4096 to avoid truncation on CJK corpora.

Changes

• Read QMD_RERANK_CONTEXT_SIZE from environment, parse as integer
• Validate range (512-32768), fall back to 2048 on invalid values
• Updated comment to document the env var

Test plan

• QMD_RERANK_CONTEXT_SIZE=4096 qmd query "test" uses 4096 context
• Default behavior unchanged without env var
• Invalid values (negative, non-numeric) fall back to 2048
• npm run build passes
• npx vitest run test/ passes

This contribution was developed with AI assistance (Claude Code).