docs: add CI and DeepWiki badges to README

[bborbe]

Summary

• Add CI workflow badge linking to GitHub Actions
• Add DeepWiki badge linking to deepwiki.com/bborbe/semantic-search

Matches the badge convention from dark-factory (CI + DeepWiki applicable; pkg.go.dev/GoReportCard are Go-only).

Test plan

• Badges render on GitHub
• CI badge reflects current workflow status

[ben-s-pull-request-reviewer]

{
  "verdict": "approve",
  "summary": "Docs-only PR adding CI and DeepWiki badges to README. Both badge URLs are valid: the GitHub Actions workflow file exists at .github/workflows/ci.yml, and the DeepWiki URL follows correct format. CHANGELOG.md is properly updated with an Unreleased entry.",
  "comments": [],
  "concerns_addressed": [
    "correctness: GitHub Actions workflow exists at .github/workflows/ci.yml, CI badge link will resolve correctly",
    "correctness: DeepWiki badge URL format is correct (deepwiki.com/bborbe/semantic-search)"
  ]
}