Add Sandcastle daemon smoke marker

[cexll]

Issue: #46

Target branch: main
Source branch: agent/issue-46
Run log: /Users/chenwenjie/workspaces/sandcastle/.sandcastle-daemon-e2e/20260605T214726Z/data/runs/cexll/codex-mcp-server/46/attempts/001.log

Summary

• Created the repository-root smoke marker file sandcastle-daemon-smoke-20260605T214726Z.md with the required marker content.
• Committed the marker file locally on branch agent/issue-46.

Validation

• node -e "const fs=require('fs'); const expected='sandcastle github-daemon smoke 20260605T214726Z'; const actual=fs.readFileSync('sandcastle-daemon-smoke-20260605T214726Z.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: Verifies the smoke marker file exists and its trimmed content exactly matches the expected string.

Review

Self-reviewed the committed change to ensure it is limited to the requested root-level marker file and validated the file content after the local commit.

Risks

• None reported

Sensitive files skipped by daemon auto-commit

• None