feat: add Slack --queue flag

[fineas-bot]

Summary

• add a non-sticky --queue Slack flag that strips itself from the prompt
• persist queued messages without steering the active harness turn, then execute them sequentially when the thread becomes idle
• preserve steering as the API default for existing clients and document the new behavior

Validation

• pnpm --filter slackbotv2 run check:types
• pnpm --filter slackbotv2 test (222 passed, 1 skipped, 0 failed)
• cargo +nightly fmt --manifest-path services/api-rs/Cargo.toml --all -- --check
• targeted centaur-api-server queue/default test passed
• cargo +stable check --manifest-path services/api-rs/Cargo.toml -p centaur-api-server -p centaur-session-runtime -p centaur-session-cli -p centaur-workflows --quiet
• cargo +nightly clippy --manifest-path services/api-rs/Cargo.toml -p centaur-api-server -p centaur-session-runtime -p centaur-workflows --all-targets -- -D warnings

Full-workspace nightly check remains blocked by pre-existing bail! macro semicolon errors in centaur-perms and centaur-session-cli.

Prompted by: greg