Add Sandcastle daemon smoke marker

[cexll]

Closes #16

Target branch: main
Source branch: agent/issue-16
Run log: /Users/chenwenjie/workspaces/sandcastle/.sandcastle-daemon-e2e/20260604-192257/data/runs/cexll/codex-mcp-server/16/attempts/001.log

Summary

• Created repository-root marker file sandcastle-daemon-smoke-20260604-192257.md with the exact requested smoke-test content.
• Committed the change locally on branch agent/issue-16.

Validation

• node -e 'const fs=require("fs"); const expected="sandcastle github-daemon smoke 20260604-192257"; const actual=fs.readFileSync("sandcastle-daemon-smoke-20260604-192257.md","utf8").trim(); if (actual !== expected) { console.error(JSON.stringify({expected, actual})); process.exit(1); }': exit 0; Command completed successfully with no output.; coverage: Verifies the marker file exists and its trimmed content exactly matches the expected Sandcastle daemon smoke string.

Review

Self-review confirmed the requested file content, validation command, and local commit were completed. No follow-up fixes were needed.

Risks

• None reported

Sensitive files skipped by daemon auto-commit

• None