initial dingger: tmux agent watcher with herdr's detection engine

extract detect-core from herdr's src/detect (engine + 22 manifests,
tests unchanged) and add the dingger binary: tmux capture-pane watcher,
process-tree agent identification, herdr's notification policy, embedded
notification sounds, @agent_state write-back, and catalog-based manifest
refresh. viewer screens are non-observations per the engine contract;
reviewed by a three-model panel (opus 5.5, gpt 6.1 sol, kimi k3) with
all confirmed findings applied as deletions.