Description 450 -> 266 chars, body 1044 -> 462 words. The bisect procedure
moves to references/bisect.md.
The boundary clause read 'not writing or rewriting commits', which
disclaimed the cherry-pick and revert this skill's own Step 3 executes; it
now excludes authoring and rebasing only. Adds 'backport' so cherry-pick is
reachable by natural language at all, accepting 266 chars against a 250
soft target and a 400 ceiling.
