docs(acp): add a Subscription Scope section to the configuration reference

[EmminiX]

Summary

Adds a "Subscription Scope" section to the buzz-acp configuration reference, covering five flags
that had no entry there: BUZZ_ACP_SUBSCRIBE, BUZZ_ACP_CHANNELS, BUZZ_ACP_KINDS,
BUZZ_ACP_NO_MENTION_FILTER and BUZZ_ACP_CONTEXT_MESSAGE_LIMIT. None of those env var names
appear anywhere in the README today, while every other group of flags has a table.

It sits after the Inbound Author Gate on purpose: that section decides whose events arrive,
this one decides which channels are watched and what opens a turn.

The Forum Channels section already uses --no-mention-filter and explains what it enables, so this
is not introducing the flag. What is missing there is its scope, which is the part that bites:
the harness subscribes to every channel the identity belongs to and the membership notification
subscription adds more over time, so in mentions mode the flag applies across all of them at
once. With several agents in one workspace and the author gate widened, one message can open a turn
on several agents.

The callout says that and points at the two things that bound it, --channels and the author gate.
I lost a day to this, which is why it seemed worth writing down.

Duplicate search

No open PR documents these flags. #4922 touches BUZZ_ACP_NO_MENTION_FILTER but is a code fix for
boolean parsing rather than documentation. Five open PRs touch this file; none edits the
configuration tables.

Testing

Documentation only, no code paths changed. Every flag name, default and behaviour statement was
checked against crates/buzz-acp/src/config.rs, specifically resolve_channel_filters
(require_mention = !no_mention_filter, applied per channel) and resolve_dynamic_channel_filter
(channels_override filters discovery rather than replacing it, and is ignored in config mode).

Rebased on current main.

Logging

No logging changes.