docs: state the accidents-not-adversaries threat model; add ROADMAP

The gates in front of destructive operations exist to stop mistakes
(stale rosters, typos, racing hosts); stopping a malicious group
member is a side effect, not the design goal. Written into README
§Security and the man page so future checks are judged by that bar.
ROADMAP.md captures the direction: social features (knock, log,
clean) over further hardening, plus the explicit resist list.

ROADMAP.md

@@ -0,0 +1,51 @@
+# Roadmap
+
+Direction, not promises. Two rules govern everything below (README
+§Security has the long form):
+
+- **Accidents, not adversaries.** New checks earn their place by
+  preventing a plausible mistake — stale state, a typo, a race — not by
+  closing a hypothetical attack from a friend.
+- **Social, not mechanical.** New features earn their place by
+  encouraging people to work together in a terminal, not by adding
+  machinery.
+
+## Next — small, high value
+
+- **`party knock <name>`** — an uninvited group member pings the host
+  via `write(1)`: "veg wants to join fiesta". `party list` already
+  shows invite-only parties; knock completes that loop. The feature
+  most likely to cause spontaneous pairing.
+- **`party log`** — per-party transcript via `tmux pipe-pane` into the
+  per-party dir (group-readable so every attendee can grab a copy);
+  `party log --stop` ends it. "What did we do last night?" for
+  collectives, and the audit trail human+AI co-work needs.
+- **`party clean`** — remove the caller's own crash leftovers (dirs
+  they own whose socket is dead), turning the manual `rm -rf` recovery
+  in host's error message into a verb.
+
+## Later
+
+- **Group ergonomics** — the default `party` group needs root and a
+  relog before anyone's first party. On boxes where everyone already
+  shares a group, `host` could offer (or default to) the caller's
+  primary group when `party` doesn't exist. Fits the trust model;
+  costs a little existence-confidentiality.
+- **One liveness primitive** — fold `is_party_alive` into
+  `party_conn_state`; two concepts where one would do.
+- **Matrix** — live validation on NetBSD and DragonFly; one
+  interactive attach/role/switch pass per release (automated coverage
+  is all non-interactive).
+
+## Resist — by design, not by neglect
+
+- **No network transport.** Same-host is the perimeter; beyond it lies
+  reinventing ssh.
+- **No per-pane ACLs.** tmux can't enforce them; faking it with hooks
+  would be a leaky abstraction. Document the limitation instead.
+- **No config files, no plugin system, no ACL syscalls.** One POSIX
+  file that runs on a 30-year spread of UNIXes is the identity; the
+  2026-04 simplification that removed per-OS ACL dispatch is not to be
+  unwound.
+- **Watch the file size.** Heavy why-comments are a feature; feature
+  growth that pushes the single file far past its current size is not.