Add tsk log task / log namespace for ref edit history
Walks the commit chain on a tsk ref and prints it newest-first in a
git-log-style oneline + author + relative-time layout. tsk log task
takes a -T tsk-N; tsk log namespace defaults to the active namespace.
The same log_ref helper covers any future per-ref view (queue, etc.).
Tests: workspace tests for both task and namespace history; lib test
covers the relative_time formatter at every breakpoint.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>