docs: the chat window, the Inbox and security groups

Three of the surfaces a user asks about most had nothing in `docs/`, so the
assistant answered from guesswork: the chat itself (its two layouts, the tab
bar and what lands on which tab, every control around the composer, the slash
commands it must never forward to the model), the Inbox (why background work
asks there rather than in the chat, and that an unanswered card is a stopped
job, not a slow one), and security groups — the honest answer to "why does it
keep asking me for permission?", including what a group is *not*: not a mode,
not a data boundary, and not advisory.

Each page states the misreadings users actually arrive with, since correcting
those is most of the work; the tables list what the person is looking at, not
what the code does.

CHANGELOG.md

@@ -51,6 +51,10 @@ release PR may merge — and a section is closed at the commit that bumps it.
 - DeepInfra, as a declarative LLM provider.
 - The project coordinator offers to keep a history of a project.
 - An agent can ask which connectors it holds instead of guessing.
+- The assistant can now explain the chat window itself (tabs, the composer's controls,
+  the slash commands), the Inbox and its three kinds of pending request, and the security
+  groups behind "why is it asking me for permission?" — ask it in plain words instead of
+  hunting through the pages.
 
 ### Changed