feat(mail): HTML lint lib + Larksuite-native autofix + lark-mail skill

[bubbmon233]

Summary

Adds an HTML lint library + Larksuite-native autofix to lark-cli mail, plus the skills/lark-mail/ skill bundle (2 reference docs, 5 HTML templates, the +lint-html shortcut, and writing-path lint integration across all 6 compose shortcuts).

What's in this PR

1. Lint library (shortcuts/mail/lint/)

3-tier rule set:

• Error: drop dangerous tags (<script> / <iframe> / <form> / <input> / <link> / <object> / <embed>), on* event handlers, and javascript: / vbscript: / file: URLs.
• Warning + autofix: rewrite HTML4-era tags (<font> / <center> / <marquee> / <blink>).
• Larksuite-native autofix: rewrite <p> / <ul> / <ol> / <li> / <blockquote> / <a> to mail-editor native markup so AI can write the simplest HTML and still produce native-quality rendering.
• Inline-style and URL-scheme allow-list filtering.
• <style> block passthrough (server adds CSS scope class).

2. +lint-html shortcut (preview / CI)

Read-only HTML preview tool. Default envelope returns only cleaned_html; --show-lint-details adds full warnings[] / errors[]. --strict exits non-zero on any finding (CI gate).

3. Writing-path lint in the 6 compose shortcuts

+send / +draft-create / +reply / +reply-all / +forward / +draft-edit body op all run lint before drafting:

• lint_applied_count / original_blocked_count — always present.
• lint_applied[] / original_blocked[] — only with --show-lint-details.
• compose_hint — points AI consumers to the HTML writing guide.

4. skills/lark-mail/ skill bundle

• 5 pre-rendered Larksuite-native HTML templates: weekly newsletter, personal weekly report, team weekly report, market research report, résumé.
• 2 reference docs:
  • references/lark-mail-html.md — writing rules + format primitives + template-usage flow.
  • references/lark-mail-lint-html.md — +lint-html usage + return-value contract + 9 examples.
• SKILL.md updates linking the new docs and templates.

5. Sealed conventions

Fixed writing conventions enforced by the lint library, the Larksuite mail-editor data model, or the upstream service-side sanitiser.

• @user mention chip — id="at-user-N" is the only hard requirement; do not write data-user-id.
• Highlight palette — 3 colors (pink milestones, yellow follow-ups, green completed); black text, no bold / padding / border-radius.
• Brand color palette — main black, 3 levels of grey, Lark blue / deep blue, alert red, emergency orange, light pink / light grey backgrounds, border grey.
• URL scheme allow-list — http(s): / mailto: / cid: / data:image/* only.
• Inline style allow-list — font-* / color / background-color / text-* / line-height / letter-spacing / vertical-align / margin* / padding* / width / height / display / border* / list-style* / white-space / word-break / overflow / transition / cursor / opacity.
• Tag allow-list — <p> / <div> / <span> / <a> / <img> / <table> (with <thead> / <tbody> / <tfoot> / <tr> / <td> / <th> / <caption> / <colgroup> / <col>) / <ul> / <ol> / <li> / <blockquote> / <h1>-<h6> / <b> / <i> / <em> / <strong> / <u> / <s> / <sub> / <sup> / <pre> / <code> / <style>.
• Writing-style floor — subject ≤ 50 chars; decision-first; lists instead of "一、二、三" / "①②③"; emoji only as status tags; greeting / sign-off ≤ 1 paragraph each.

Tests

• shortcuts/mail/lint/... — unit tests for every rule.
• shortcuts/mail/mail_lint_html_test.go — +lint-html envelope contract.
• shortcuts/mail/mail_lint_writepath_test.go — writing-path envelope contract.
• 5 templates verified via +draft-create smoke test.

Test plan

• go test ./shortcuts/mail/lint/... ./shortcuts/mail/...
• All 5 templates render correctly via +draft-create smoke
• Default vs --show-lint-details envelope verified for both +lint-html and +draft-create