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/bisect.md

@@ -0,0 +1,68 @@
+---
+topic: bisect
+source_keys:
+  - git-scm-bisect-docs
+---
+
+# Finding a commit with `git bisect`
+
+Read this when the question is *which commit changed the behaviour* and there is no string, file,
+or line range to search the log for. Binary search reduces the trials from O(N) to O(log N).
+
+## Manual flow
+
+```bash
+git bisect start
+git bisect bad [HEAD]        # mark current (or specified) as broken
+git bisect good <commit>     # mark known-good baseline
+# Git checks out the midpoint; test it
+git bisect good              # test passes
+git bisect bad               # test fails
+# Repeat until git reports "X is the first bad commit"
+git bisect reset             # return to the original HEAD
+```
+
+## Automated
+
+With a test command available, use `git bisect run <cmd>`. Git reads the exit code: `0` good,
+`1`–`124` bad, `125` skip (build broken), `126`–`127` POSIX shell errors, treated as bad, and
+`128` or above aborts the session outright rather than marking the commit bad.
+
+## Untestable commits
+
+`git bisect skip` excludes a commit that cannot be built or tested without deciding good or bad
+for it. When the first bad commit is adjacent to a skipped range, bisect reports that it cannot
+pinpoint the culprit and lists the candidates — that is the precise answer the skip range allows,
+not a failure.
+
+## Undoing a wrong good/bad call
+
+`git bisect log` prints the session's decision history. Save it, edit out the mistaken entry, and
+resume from the corrected log rather than restarting the search:
+
+```bash
+git bisect log > bisect.log
+# edit bisect.log, removing the wrong decision
+git bisect reset && git bisect replay bisect.log
+```
+
+## Narrowing and speeding up
+
+- `git bisect start HEAD v1.2 -- src/` restricts bisection to a path, cutting the trial count.
+- `--no-checkout` updates the `BISECT_HEAD` ref instead of checking out a working tree — useful
+  for tests that do not need one, and automatic in bare repos.
+- `--first-parent` follows only first parents at merges, finding the integration commit that
+  introduced a regression while ignoring broken side branches.
+
+## Inspecting the remaining candidates
+
+`git bisect visualize` (alias `view`) opens the suspects in gitk, falling back to `git log` when
+no graphical display is detected. Add `--stat` or `-p` for a diffstat or full patches.
+
+## Hunting a non-bug property change
+
+`git bisect start --term-new <new> --term-old <old>` searches for any property change — a
+performance regression, say — instead of a bug. Use the custom terms in place of `good` and `bad`
+for the rest of the session.
+
+Once the first bad commit is identified, return to Step 3 to act on it and Step 4 to report it.