feat(tools/mail-source/mbox): land concrete CLI wiring for the mbox stub

[potiuk]

The mbox / local-archive mail-source adapter is currently a documentation stub. Land the concrete CLI per tools/mail-source/mbox/README.md. Read-only by construction — no drafts, no writes to the archive file.

Operations to implement:

• list_recent_threads(list, since) — parse mbox, group messages by References: / In-Reply-To:, filter by Date:
• read_thread(thread_id) — return full message history for the thread root
• thread_url(thread_id) — build archive deep-link if the adopter has a public archive (PonyMail / Hyperkitty / Pipermail), else file://

Format sniffing should accept: a single mbox file, a Maildir/ directory, or a directory of .eml files.

Why:

• Forensics / late triage — the inbox is gone but a snapshot exists.
• Air-gapped triage — a thumbdrive snapshot of security@ for an audit window.
• Compliance / discovery exports — a regulator-requested mbox export, processed through the same skills without touching live credentials.

Reference:

• Contract: tools/mail-source/contract.md
• Stub spec: tools/mail-source/mbox/README.md
• Python mailbox (stdlib) handles mbox + Maildir + MH + Babyl