perf: optimize secret-digger-claude token usage (#1894)

Address recommendations from #1889 (Claude token optimization report):

1. Disable GitHub MCP tools (github: false)
   - 52 unused GitHub MCP tools were loaded (~26K tokens/run)
   - Workflow only uses bash, cache-memory, and safe-outputs
   - Estimated savings: ~64% cost reduction ($0.51 → ~$0.18/run)

2. Add max-turns: 8
   - Claude engine supports max-turns (unlike Copilot)
   - Enforces hard turn budget to prevent runaway costs

3. Reduce timeout-minutes from 30 to 15
   - Typical runs complete in ~5 minutes with 3 turns
   - Halves the max cost ceiling for failure scenarios

4. Remove duplicate context from user message
   - Repository, Run ID, Workflow, Engine lines already injected
     by gh-aw framework into system context

Closes #1889

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

Author: lpcox