Add compact channel bot toolbar

[wesbillman]

Summary

• add a compact channel header cluster with member and bot counts plus add/manage actions
• open a dialog from the add button so users can pick a runtime, edit the default name, and set a bot prompt
• create channel-ready managed agents with the right defaults, auto-add them to the channel, and spawn them immediately

Testing

• source ./bin/activate-hermit && cd desktop && pnpm typecheck
• source ./bin/activate-hermit && cd desktop && pnpm check
• pre-push hooks: cargo fmt --all -- --check, cd desktop && pnpm build, cargo check --manifest-path desktop/src-tauri/Cargo.toml, cargo clippy --workspace --all-targets -- -D warnings, ./scripts/run-tests.sh unit