fix: increase security-guard max-turns from 15 to 25

The security guard agent was hitting the 15-turn limit on large PRs
(e.g., 29-file workflow recompiles with ~979KB diffs). The agent spends
several turns handling oversized MCP payloads before falling back to
alternative strategies, leaving insufficient turns to complete the review.

Increasing to 25 turns gives adequate headroom for large diffs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: lpcox