feat(commands): add /group to manage reply policy from chat - #5974

⚠️ Depends on #5973

Review and merge #5973 first. This branch is stacked on it, so until #5973
lands this PR's diff also contains #5973's commits.

Once #5973 is merged, this PR reduces to just the command, the store, and their
tests. I will rebase onto main as soon as #5973 lands — the diff above is not
what merges.

Note on mechanics: this repo does not have GitHub's native stacked-PR feature
enabled, and GitHub rejects a cross-fork base branch, so the stack is expressed
through this PR-to-PR dependency rather than a stacked base. Rebased on main
the change is exactly:

• nanobot/group_policy/ (store)
• nanobot/command/builtin.py (/group)
• nanobot/channels/telegram/runtime.py (resolve through the store)
• docs/chat-commands.md
• tests/test_group_policy.py

Summary

Editing channels.<channel>.groupPolicyOverrides in config.json requires touching JSON and restarting the gateway. For the common case — a topic is noisy, quiet it now — that is the wrong tool: the person seeing the noise is in the chat, and a restart drops context.

Add a /group command backed by a small runtime store, so an operator can adjust the current chat or topic from where they are.

Refs #5972.

Command

/group              → show the effective policy for this chat/topic
/group mention      → only reply here when mentioned
/group open         → reply to every message here
/group reset        → clear the override for this chat/topic

The command always acts on the calling chat/topic (from chat_id and message_thread_id), so a user never has to know or type a raw thread id.

How it layers with config

Runtime overrides supplement the static config rather than replace it, so durable policy can stay in config.json while /group handles exceptions. Most specific scope wins:

1. runtime topic override
2. runtime chat override
3. static topic override (config.json)
4. static chat override (config.json)
5. channel-wide groupPolicy

Scopes are keyed by channel as well (telegram:<chat_id>[:<thread_id>]), so Telegram and Discord overrides cannot collide.

Storage

nanobot/group_policy/ mirrors the existing nanobot.pairing.store pattern deliberately — JSON file under the data dir, process lock, atomic writes, no external database — because it is the same shape of problem (small per-scope settings, mutated from chat, read on every inbound message):

<data_dir>/group_policy.json
{"overrides": {"telegram:-100:7819": "mention"}}

_load() validates on read so a corrupted file degrades to "no overrides" instead of raising on every message, while a transiently unreadable file propagates — mirroring how pairing/store.py avoids persisting an empty view over real data.

Changes

• nanobot/group_policy/ — store + resolve_policy() used by both the command and the Telegram channel.
• nanobot/command/builtin.py — /group handler and palette entry.
• nanobot/channels/telegram/runtime.py — resolves through resolve_policy(); /group added to the forwarded command list.
• docs/chat-commands.md — command table.

Tests

New tests/test_group_policy.py (13 tests), using an isolated temp data dir:

• default policy when nothing is set
• static chat override; static topic beats static chat
• runtime override beats static config
• runtime topic beats runtime chat, and siblings keep the chat value
• no leakage across chats (same thread id) or across channels
• reset clears one scope and leaves the parent intact
• unsupported policy rejected; list filters by channel; overrides persist to disk

$ pytest tests/test_group_policy.py tests/channels/ -q
309 passed

Notes for Reviewers

• The command replies with render_as: "text" and only acts in group scopes; in a DM it explains that it applies to groups.
• The effective-policy lookup reads the same values the channel uses, so /group with no argument reports what the next message will actually get — including overrides set via config.json.
• Full suite has the same 4 pre-existing environment failures as main (missing local rg/poppler), unrelated to this change.
• If feat(telegram): per-chat and per-topic group policy overrides #5973 is rejected in favor of a different key format, only the static_overrides lookup in resolve_policy() needs adjusting; the store and command stay as-is.