ci(claude-bot): tune models, Copilot-style PR review, issue research mode

- handle-issue: use Sonnet 4.6 and raise max-turns 150 to 250

- handle-pr: use Opus 4.8; rewrite review as inline comments stating the problem plus a suggestion block, posted as one COMMENT review

- mention: use Opus 4.8; on issues do research only (never commit) with full comment/history context and feature-request feasibility analysis; PR commit-on-request behavior unchanged

- reformat the mention append-system-prompt into a readable multi-line block (verified it still parses as a single CLI argument)