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

The mbox / local-archive mail-source adapter is currently a documentation stub. Land the concrete CLI per [`tools/mail-source/mbox/README.md`](../tree/main/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`](../tree/main/tools/mail-source/contract.md)
- Stub spec: [`tools/mail-source/mbox/README.md`](../tree/main/tools/mail-source/mbox/README.md)
- Python `mailbox` (stdlib) handles mbox + Maildir + MH + Babyl