chore: ignore prolog-mcp session directory

The prolog-mcp MCP server creates ./prolog-sessions relative to its
cwd, which is this repo root when Claude Code launches it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UNMyVJTwXdcv369x98H98b

.gitignore

@@ -28,3 +28,6 @@ proptest-regressions/
 .claude/worktrees/
 *~
 /roadmap.html
+
+# prolog-mcp session files (server writes ./prolog-sessions relative to cwd)
+prolog-sessions/