docs: qualify the 256-channel figure — default policy knob, not a wire limit

channel_id is u32 on the wire (~4B ids, monotonic counter, ADR-040);
256 is the default per-connection max_channels (memory bound, 256x1MiB
reassembly buffers) and the default per-identity DoS cap (ADR-041) —
both configurable without a wire change. AGENTS.md convention 10 and
the OQ-SK-03 composed-egress note now carry the qualifier.

AGENTS.md

@@ -161,7 +161,10 @@ implementation agents.
    (assembly layer), never the protocol crate.
 
 10. **Backpressure and limits are inherited, not redefined** — bounded
-    per-channel buffers, the 256-channel per-connection cap, monotonic
+    per-channel buffers, the 256-channel default caps (per-connection
+    memory bound, ADR-040; per-identity DoS defense, ADR-041 — policy
+    knobs, configurable without a wire change; the `u32` wire space
+    itself is ~4 billion), monotonic
     channel IDs, and the zero-length-sentinel EOF convention are
     alkcall channels invariants (see alkcall ADR-040/041/034). The
     crate consumes them via `ChannelCore`/`ChannelClient`; do not build