feat(marmot-interop): add zero-prompt headless harness driving amy

Matches the 13 scenarios in marmot-interop.sh but runs A via the new
`amy` CLI instead of prompting the human. Identities B and C still go
through `wn`/`wnd` from whitenoise-rs.

Layout:
- marmot-interop-headless.sh    — top-level entrypoint
- headless/setup.sh             — preflight, daemon lifecycle, identities
- headless/helpers.sh           — amy wrappers + assertion helpers
- headless/tests-create.sh      — tests 01..05
- headless/tests-manage.sh      — tests 06, 07, 08, 11
- headless/tests-extras.sh      — tests 09, 10, 12, 13

Reuses lib.sh for colours, record_result, wait_for_invite/message,
jq_group_id, and dump_daemon_diagnostics.

Keeps the interactive marmot-interop.sh intact for final UI sign-off.

https://claude.ai/code/session_01M6dCKAF5Y1VyHGZPjzwDXq

tools/marmot-interop/.gitignore

@@ -1 +1,2 @@
 state/
+state-headless/