[Dev10x:gh-pr-monitor] Update detect step to prefer MCP tool

Problem

Step 1 of gh-pr-monitor documents gh-pr-detect.sh as the PR context detection mechanism. The agent correctly used mcp__plugin_Dev10x_cli__pr_detect instead (improvement), but the documentation should reflect this preference.

Evidence

Agent used MCP tool pr_detect instead of documented script. This was assessed as an improvement — MCP tool is cleaner and listed in the skill's allowed-tools.

Proposed Fix

Update Step 1 to list mcp__plugin_Dev10x_cli__pr_detect as primary, with the shell script as fallback. This aligns with the broader pattern of preferring MCP tools over shell scripts across all Dev10x skills.