[APTSPC]

Send outbound replies with an HTML body (multipart/alternative)

What this changes

AgentMail replies are delivered as text/plain only. The provider's reply API already accepts an
html body — this PR uses it, passing a rendered version of the same Markdown the agent produced:

const text = (ctx.payload?.text ?? ctx.text)?.trim() || undefined;
const html = text ? renderEmailHtml(text) : undefined;
...
{
  ...(text ? { text } : {}),
  ...(html ? { html } : {}),
  ...
}

A new dependency-free renderer (src/channel/src/markdown-html.ts) covers what agent replies
actually contain: headings, bold/italic/strikethrough, inline code, fenced code, links (explicit and
bare URLs), bullet/numbered lists, pipe tables, blockquotes, horizontal rules and paragraphs.

Why

Agent replies are Markdown. In any HTML-capable client that means recipients currently see literal
**bold**, - item and | table | syntax — an email that looks broken rather than formatted.
Supplying both parts is the standard fix: HTML clients render the formatted version, text-only
clients and archives keep the readable plain-text alternative.

Safety / compatibility

• text is still sent in every case, so this is additive — text-only clients, mail archives and
  providers that strip HTML are unaffected.
• Attachment-only replies are unchanged (html is only built when there is text).
• No new runtime dependency; the renderer is string handling only (no marked, no DOM).
• All user/agent content is HTML-escaped before any markup is applied, and links are restricted to
  http(s) URLs. Raw input like <script> is escaped, not passed through.

Rendering notes

• The document uses a role="presentation" table shell with text-align:left and no
  max-width + margin:0 auto wrapper: Gmail honours those two properties and centres the entire
  message body, which reads wrong for a business email. An optional maxWidth option anchors a
  column to the left edge instead.
• Inline styles only (no <style> block), since several clients drop head CSS.
• 720px-class layouts were deliberately avoided for the same reason.

Testing

• src/channel/src/markdown-html.test.ts covers headings/emphasis/lists, escaping, autolink
  punctuation, explicit links, inline code, pipe tables, fenced code, paragraph line breaks, the
  left-aligned shell, and the optional max width.
• npm test (vitest) — new suite only; no existing tests touched.
• Manually verified end to end against a live AgentMail inbox: the reply arrives with both parts,
  Gmail renders the HTML and the plain-text alternative is intact.

Follow-ups (not in this PR)

• Optionally expose linkColor / maxWidth through channel config.
• If maintainers prefer opt-in behaviour, the renderer can be gated behind a htmlFormat
  config enum defaulting to "text"; happy to add that if it is the preferred shape.