test(cli): amy↔amy NIP-17 DM interop harness

Adds `tools/marmot-interop/dm-interop-headless.sh` — a zero-prompt
end-to-end harness that runs two independent `amy` processes against
a loopback nostr-rs-relay and verifies the NIP-17 DM surface.

Six scenarios:

  dm-01  text round-trip (kind:14) in both directions
  dm-02  `dm list` surfaces prior exchange with `type:text` discriminator
  dm-03  strict kind:10050 refuses sends to an inboxless recipient
         (surfaces the `no_dm_relays` error JSON)
  dm-04  `--allow-fallback` opts into NIP-65 read / bootstrap chain
         (asserts a `relay_source: "bootstrap"` or `"nip65_read"` row)
  dm-05  file message reference mode (kind:15) — send-file with
         --key / --nonce, verify recipient decodes url + key + nonce + mime
  dm-06  no-flag `dm list` advances `state.giftWrapSince`; second call
         returns an empty `messages` array

Shape matches the existing Marmot harness: reuses `lib.sh` for logging
+ result tracking, reuses `setup.sh` for the nostr-rs-relay lifecycle
(`start_local_relay` / `stop_local_relay`), adds a slim `setup-dm.sh`
preflight (amy + relay only, no whitenoise-rs / Marmot patches), and
defines tests in `tests-dm.sh`.

No new CI job yet — the relay build needs Rust + ~3 minutes on a
cold cache, same constraint as the Marmot harness.

Upload-mode (`dm send-file --file PATH --server URL`) is not scripted
here: it needs a local Blossom server, which is out of scope for this
pass. The shared upload classes are unit-tested on desktop at
`desktopApp/src/jvmTest/kotlin/.../service/upload/`.

cli/ROADMAP.md

@@ -94,7 +94,12 @@ move anything, re-audit — you're probably duplicating logic.
 7. **`amy zap send|verify`** (NIP-57).
 8. **Distribution** — Homebrew + Scoop + `.deb` in the same release
    pipeline as desktop. Plan: `cli/plans/2026-04-21-cli-distribution.md`.
-9. **Test suite** — end-to-end against a local relay.
+9. **Test suite** — end-to-end against a local relay. Marmot interop is
+   covered by `tools/marmot-interop/marmot-interop-headless.sh`; NIP-17
+   DM interop between two `amy` clients is covered by
+   `tools/marmot-interop/dm-interop-headless.sh` (text + file + strict
+   10050 + fallback + cursor-advance). Neither runs in CI yet (both
+   need Rust + ~3 min cold relay build).
 10. **Everything else in the matrix.**
 
 ---