Extend reactions reference with `merge_conflict` and `bot_review`

[sergeyklay]

The reactions reference page created in M20 (#470) documents ci_failure, review_comments, and auto_merge. M21 adds two more reaction kinds that must be covered in the same reference: merge_conflict (#416) and bot_review (#415). The page must be updated rather than fragmented into per-reaction pages.

Scope: add a section for reactions.merge_conflict covering fields (max_retries default 1, escalation) and the detection/dispatch flow (conflict detected on reconcile tick, transition from no-conflict to conflict triggers single dispatch, tracking resets on resolution). Add a section for reactions.bot_review covering bot-author detection (user.type == "Bot", configurable allowlist), the separation from human review_comments, and the more aggressive retry posture appropriate for deterministic linter output. Update the shared reaction model diagram if one exists, and make sure the reaction comparison table at the top of the page includes all five kinds.

Depends on #416 (merge-conflict implementation) and #415 (bot-review implementation).

Verify: reference/reactions.md on docs.sortie-ai.com covers all five reaction kinds with field tables and example WORKFLOW.md snippets. The page remains a single reference, not fragmented.