Fix Slack adapter to parse app_mention events

[mdrissel]

Summary

This adds support for parsing the app_mention event type to the Slack Socket Mode adapter, injecting a was_mentioned flag into the metadata.

Without this, the bot ignores messages when it is purely @mentioned without regular message context.

Changes

• crates/openfang-channels/src/slack.rs - add app_mention to parsed events and inject a was_mentioned flag into metadata.
• crates/openfang-cli/src/main.rs, crates/openfang-runtime/src/kernel_handle.rs - Fixed two unrelated clippy warnings on upstream main that caused linting to fail.

Backward compatibility

• Adds to existing parser matching and metadata logic, keeping existing message handling intact.

Checklist

• cargo fmt
• Tested locally via cargo test --workspace and cargo clippy --workspace --all-targets -- -D warnings
• No new unsafe code
• No secrets or API keys in diff