Matrix: add groupPolicyOverrides (per-room group policy, parity with Napcat)

Problem

groupPolicy is channel-wide on Matrix (one of open, mention, allowlist applies to every group room). Only DMs (≤2 members) bypass it (_should_process_message, nanobot/channels/matrix/runtime.py, v0.3.5).

A deployment with many rooms usually wants different behavior per room:

This cannot be expressed today: open everywhere is noisy in the shared room, mention everywhere silences the topic rooms.

Existing precedent in the codebase

The Napcat channel already ships this exact mechanism — the ask is a small parity port to Matrix:

• nanobot/channels/napcat/manifest.py: "groupPolicyOverrides": field("json", default={})
• nanobot/channels/napcat/runtime.py:

  group_policy_overrides: dict[str, GroupPolicy] = Field(default_factory=dict)
  policy = self.config.group_policy_overrides.get(str(group_id), self.config.group_policy)

Related Telegram-side RFCs with the same gap: #3309, #5972.

Proposed config

{
  "channels": {
    "matrix": {
      "groupPolicy": "open",
      "groupPolicyOverrides": {
        "!aaaa...:example.org": "mention"
      }
    }
  }
}

• Keys: Matrix room IDs. Values: the same vocabulary as Matrix's groupPolicy — "open" | "mention" | "allowlist" (Napcat additionally accepts a probability float; not required here).
• Precedence: per-room override → channel groupPolicy. Rooms not listed behave exactly as today, so existing configs are unaffected.
• An "allowlist" override would reuse the existing channel-wide groupAllowFrom list for that room (minimal change; per-room allow lists could come later).
• allowRoomMentions semantics unchanged.

Implementation

Single lookup change at the existing policy-resolution point in nanobot/channels/matrix/runtime.py, _should_process_message() (~L1136–1149 in v0.3.5):

-        policy = self.config.group_policy
+        policy = self.config.group_policy_overrides.get(room.room_id, self.config.group_policy)

plus the matching field("json", default={}) declaration in channels/matrix/manifest.py, mirroring Napcat.

Happy to attempt the PR if this is wanted.