fix(acp): stop SubscribeMode::All from subscribing to every event kind

[BradGroux]

What users saw

With BUZZ_ACP_SUBSCRIBE=all, typing indicators (kind 20002) could open agent turns even though the user had not sent a message. Those empty ephemeral events consumed work and could cancel a pending scheduled wakeup before the actual message arrived.

Why it happened

SubscribeMode::Mentions defaults to an explicit kind list: stream messages (9), workflow approval requests (46010), and reminders (40007).

SubscribeMode::All left the kind list empty when BUZZ_ACP_KINDS was unset. Empty means wildcard in both the relay filter and the in-process subscription rule, so all expanded from all relevant messages to all event kinds.

What changed

Added one default_subscription_kinds helper and use it everywhere the default subscription is constructed:

• initial relay channel filters
• dynamically discovered channel filters
• the in-process SubscriptionRule for both mentions and all

All now means the same bounded event kinds as Mentions, without the mention requirement. Operators can still supply a custom list with BUZZ_ACP_KINDS.

How this was tested

Updated the initial-filter regression test and added dynamic-filter coverage. The assertions compare the exact default list and confirm that require_mention is false. Existing runtime-rule tests cover construction of the in-process rules.

Validation at 3aeeb443a:

./bin/cargo fmt --all -- --check
env -u BUZZ_ACP_LAZY_POOL cargo test -p buzz-acp --lib
cargo clippy -p buzz-acp --all-targets -- -D warnings

All 674 buzz-acp library tests passed, and formatting and strict Clippy checks completed successfully. The test command removes an ambient lazy-pool override because that variable intentionally changes the default-config tests.

Scope and non-goals

• Does not change SubscribeMode::Config.
• Does not remove or reinterpret an explicit BUZZ_ACP_KINDS override.
• Does not change reply threading for ephemeral events.

Closes #4949.

[BradGroux]

Review found a second wildcard seam beyond the relay filters: the in-process SubscriptionRule for SubscribeMode::All still used an empty kind list, which also means all kinds. I pushed 3aeeb443a so initial filters, dynamic filters, and runtime rules all use one shared bounded default.

At that head, rustfmt, all 674 buzz-acp library tests, and strict Clippy pass. The package test was run without an ambient BUZZ_ACP_LAZY_POOL override because that setting intentionally changes default-config behavior.