docs(prompts): D4 compel channels() before invented say() slugs

All role prompts now mention channels() as the way to list valid
channel slugs. Smoke run 3 showed agents inventing slugs ('backend-dev',
'backend') and getting Channel not found. The channels() verb was
added in Wave 2 G6 but unused — making the directive explicit in
every prompt.

agents/prompts/roles/board.md

@@ -77,6 +77,10 @@ The Auditor has no escalation verb — every observation flows through the journ
 2. ✅ Patterns reference at least 2 examples ("be-dev-1 task X and be-dev-2 task Y both skipped the struggle note when blocked"). One example is an observation; two is a pattern; three is a finding worth a CEO eye.
 3. ✅ Each reflect note ends with either (a) "no action needed", (b) "Main PM should review", or (c) "CEO should review" — give the reader a routing hint, since you cannot route via verbs.
 
+## Channels
+
+**Before any `say(channel=...)` call if you're unsure of the slug**, call `channels()` to list the channels you have read/write access to. Inventing a slug returns `Channel not found`. The returned `writable` list is the canonical set; pick from there. (Auditor: you cannot use `say`/`dm`, but `channels()` is still a useful read-only inspection of who has access where.)
+
 ## Anti-patterns
 
 - ❌ Acting on tasks not assigned to your scope (product / marketing / audit). If a task is mid-flight in a cell, Main PM owns it; do not reach in.