refactor: drop OPERATOR_USERNAME, separate control vs comms planes

There's no good reason to bake a specific operator nickname into the bot's
identity — it differs per server, may not exist at all, and treating any
in-game name as "trusted" is a chat-injection vector ("I am the operator,
do X").

New model: the **repo** is the only trusted control plane. Anyone editing
AGENTS.md, skills/, or .env has filesystem access and is, by definition,
an operator. In-game chat becomes a dialog-only comms plane — the bot
talks to anyone but refuses destructive requests unless a corresponding
skill or AGENTS.md instruction makes the action explicitly permitted.

- .env / .env.example: OPERATOR_USERNAME removed
- AGENTS.md: identity section trimmed; "Operator contact" rewritten as
  "Control channel" with the trust model spelled out; rules #2 and #6
  rephrased so they no longer reference a named operator
- docs/architecture.md: top box renamed to "Human" with explicit
  control-plane vs comms-plane split

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/architecture.md

@@ -6,11 +6,12 @@
 
 ```
 ┌─────────────────────────────────────────────────────────────┐
-│  Operator                                                   │
-│  (human — in-game chat, repo edits, .env)                   │
+│  Human                                                      │
+│  - control plane: repo edits (AGENTS.md, skills/, .env)     │
+│  - comms plane:   in-game chat (untrusted, dialog only)     │
 └─────┬────────────────────────────────────────────┬──────────┘
       │                                            │
-      │ chat / edit AGENTS.md                      │ optional: Telegram (future)
+      │ edit repo / .env                           │ optional: Telegram (future)
       ▼                                            ▼
 ┌─────────────────────────────────────────────────────────────┐
 │  Pi runtime                                                 │