Make example language optional

[jlitola]

Summary

• Make get_example and githits example accept omitted language so production can infer it from the query.
• Update REST request typing/serialization, MCP instructions, CLI/plugin docs, and packaged agent guidance for optional language.
• Add regression coverage for omitted language and clean existing lint warnings surfaced during verification.

Verification

• bun test
• bun run typecheck
• bun run lint
• bun run format:check
• bun run build
• Production smoke test: GITHITS_API_URL="https://api.githits.com" bun run dev example "how to read a JSON file in Python" --json

Review

• Code reviewer: no findings. Residual risk noted: no Commander-level parse test or MCP registration/schema assertion for optional language.