Add Sandcastle daemon smoke marker

[cexll]

Issue: #70

Target branch: main
Source branch: agent/issue-70
Run log: /Users/chenwenjie/workspaces/sandcastle/.sandcastle-daemon-e2e/20260606T081216Z/data/runs/cexll/codex-mcp-server/70/attempts/001.log

Summary

• Created the repository-root smoke marker file with the required Sandcastle daemon content.
• Committed the marker file locally on the source branch.

Validation

• node -e "const fs=require('fs'); const expected='sandcastle github-daemon smoke 20260606T081216Z'; const actual=fs.readFileSync('sandcastle-daemon-smoke-20260606T081216Z.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: Daemon-owned validation rerun after the final local commit in the locked-down Docker validation sandbox.

Review

Self-reviewed the committed change and confirmed the requested marker file is cleanly committed; unrelated pre-existing untracked sandbox dotfiles were left untouched.

Risks

• None reported

Sensitive files skipped by daemon auto-commit

• None