fix(email): add <hr> to HTML body detection tag list

[dizhaky]

Summary

Adds <hr> to _HTML_TAGS in gateway/platforms/email.py. The <hr> void element is commonly used as a horizontal divider in HTML emails; without it, emails whose only HTML marker was <hr> were classified as plain text and sent without the multipart/alternative wrapper.

Supersedes #38 (had unsigned commits; rewritten with a signed commit).

Test plan

• Send an HTML email containing only <hr> as its HTML indicator — should be detected as HTML and sent as multipart/alternative
• Plain-text emails remain plain-text (no regression)
• CI green

🤖 Generated with Claude Code

---

Note

Low Risk
Single-tag addition to an existing HTML-detection regex; no auth, SMTP, or IMAP logic changes.

Overview
Adds hr to the _HTML_TAGS pattern used by _HTML_BODY_RE / _is_html_body in the email gateway.

Bodies that only include a horizontal rule (e.g. <hr> or <hr/>) are now treated as HTML instead of plain text, so _attach_body keeps the markup in the HTML part and strips it for the plain alternative rather than escaping the whole body as preformatted text.

^{Reviewed by Cursor Bugbot for commit ad0a3b3. Configure here.}

[chatgpt-codex-connector]

You have reached your Codex usage limits for code reviews. You can see your limits in the Codex usage dashboard.

[github-actions]

🔎 Lint report: dan/email-hr-tag-detection vs origin/main

ruff

Total: 0 on HEAD, 0 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 0 pre-existing issues carried over.

ty (type checker)

Total: 8649 on HEAD, 8649 on base (➖ 0)

🆕 New issues: none

✅ Fixed issues: none

Unchanged: 4570 pre-existing issues carried over.

Diagnostics are surfaced as warnings — this check never fails the build.