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

[BradGroux]

Note: This is a replacement for PR #4974, which was accidentally closed when branches were force-pushed after a commit identity rewrite. The changes are identical, rebased onto the latest main.

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]

This is a replacement for #4974, which was accidentally closed when branches were force-pushed after a commit identity rewrite. Prior review comments and feedback are preserved on the old PR:

#4974

[BradGroux]

Rebased onto the latest main (was 36 commits behind). Both commits applied cleanly. SubscribeMode::All still leaves the default kind list empty when BUZZ_ACP_KINDS is unset, causing it to subscribe to all event kinds on current main, so this fix is still needed.

No review comments on this PR. CI will verify the build and test gate.