fix(slack): acknowledge accepted inbound messages

[taariq]

Summary

• add a bounded reactions.add acknowledgement for accepted Slack ingress
• preserve the Slack message timestamp independently from client-generated IDs
• keep reaction failures fail-open with content-free diagnostics and idempotent retry handling
• document the required reactions:write bot scope

Validation

• cargo check --workspace --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked --no-fail-fast
• cargo test --doc --workspace --locked --no-fail-fast
• cargo fmt --all -- --check
• release build and JSON-RPC capability smoke test

Closes #1