Add Sandcastle daemon smoke marker

[cexll]

Closes #19

Target branch: main
Source branch: agent/issue-19
Run log: /Users/chenwenjie/workspaces/sandcastle/.sandcastle-daemon-e2e/20260604-204551/data/runs/cexll/codex-mcp-server/19/attempts/001.log

Summary

• Created the repository-root smoke marker file sandcastle-daemon-smoke-20260604-204551.md with the exact requested content.
• Committed the change locally on branch agent/issue-19.

Validation

• node -e "const fs=require('fs'); const expected='sandcastle github-daemon smoke 20260604-204551'; const actual=fs.readFileSync('sandcastle-daemon-smoke-20260604-204551.md','utf8').trim(); if (actual !== expected) { console.error(JSON.stringify({expected, actual})); process.exit(1); } console.log('marker content matches');": exit 0; marker content matches; coverage: Confirms the smoke marker file exists and its trimmed content exactly matches the requested marker string.

Review

Self-reviewed the created file, validated the exact content before and after the local commit, and confirmed the working tree is clean.

Risks

• None reported

Sensitive files skipped by daemon auto-commit

• None