feat(render): compact search results and bounded page contents - #49

This adds `RenderOptions` with `render_text_with`/`render_markdown_with` (sources compaction, configurable excerpt and contents caps), an Exa `contents_max_chars` provider setting that rides as `max_characters`, and role-aware page/citation caps in normalization. Each behavioural branch has a test asserting the actual output — request bodies, rendered strings, and per-page char counts — so a regression would surface. The changes look sound and consistent with the repository's test-layout rules.

No findings.