refactor(git-history): retrofit to the ADR-0020 context contract

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.

plugins/git/.apm/skills/git-history/references/sources.md

@@ -12,7 +12,7 @@ Git bisect documentation covering binary search through commit history to find t
 
 - **Research doc:** plugins/git/docs/research/docs/git/history-inspection.md
 - **Doc heading:** `## git bisect`
-- **Contributing files:** SKILL.md
+- **Contributing files:** SKILL.md, references/bisect.md
 
 ## git-scm-log-docs
 
@@ -28,4 +28,4 @@ Git diff documentation covering output control (--stat, --name-only, --name-stat
 
 - **Research doc:** plugins/git/docs/research/docs/git/history-inspection.md
 - **Doc heading:** `## git diff — Output Control`
-- **Contributing files:** references/git-log-format.md
+- **Contributing files:** SKILL.md, references/git-log-format.md