feat(tools/mail-source/mailman3): add Mailman 3 / Hyperkitty archive backend #306

Add a new tools/mail-source/mailman3/ adapter implementing the mail-source contract against a public Mailman 3 archive (the Hyperkitty UI + the underlying Mailman REST API).

Operations to claim:

• list_recent_threads(list, since) — Hyperkitty exposes /list/<list>@<domain>/latest and per-month archives in machine-readable form
• read_thread(thread_id) — Hyperkitty thread page is keyed by thread root Message-ID
• thread_url(thread_id) — Hyperkitty URL by Message-ID hash
• thread_id_kind: rfc5322-message-id (matches IMAP + mbox)

Drafts / sent / write ops are unsupported — same shape as the PonyMail adapter (archive viewer only, no outbound).

Why: Hyperkitty is the standard archive UI for Mailman 3, which is what Python uses (mail.python.org), Fedora (lists.fedoraproject.org), Mailman itself, NumPy/SciPy mailing lists, the GNU project, Plone, and many smaller OSS projects. It's the non-ASF equivalent of PonyMail. Without it, every non-ASF adopter that lives on Mailman 3 is forced into Gmail-only territory.

Reference:

• Hyperkitty REST: https://docs.mailman3.org/projects/hyperkitty/en/latest/api.html
• Existing archive-viewer adapter shape: tools/ponymail/ — read-only, no drafts
• Contract: tools/mail-source/contract.md