Python: Improve PR template and breaking-change label automation (#6473)

* Improve PR template and breaking-change label automation

- Add a structured "Related Issue" section using GitHub closing keywords
- Add a Review Guide prompt (major changes, impact, reviewer focus) with a
  note that the focus item is for human reviewers only
- Add checklist items for issue linkage / no duplicate PRs and invert the
  breaking-change item (checked = not breaking)
- Extend label-title-prefix to prepend [BREAKING] when the "breaking change"
  label is added
- Add label-breaking-change workflow to apply the "breaking change" label
  when a PR title contains [BREAKING]

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Add pull-requests agent skill with dotnet/python links

- Add root .github/skills/pull-requests/SKILL.md covering PR description
  authoring (following the PR template) and the review-comment workflow
  (review -> plan -> user review -> implement -> reply to all -> resolve)
- Symlink the skill from python/.github/skills and dotnet/.github/skills
- Reference the skill from python/AGENTS.md and dotnet/AGENTS.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Fold breaking-change labeling into label-pr workflow

Move the title -> 'breaking change' label logic into the existing label-pr
workflow (which already applies the python/.NET labels) and drop the separate
label-breaking-change workflow.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR title prefix review feedback

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Pin patched MessagePack for .NET restore

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Revert MessagePack central pin

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Move title prefix tests out of tracked GitHub tests

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Exclude skill docs from CI path filters

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Match skill symlinks in CI path exclusions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Exclude AGENTS docs from CI path filters

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Scope title-prefix normalization to a real prefix

The normalization branch in addTitlePrefix matched ^Python (no colon), so
titles like "Python samples improvements" or "Pythonic refactor" were treated
as already-prefixed and only re-cased, never receiving the "Python: " prefix.
Scope the match to ^<prefix>:\s* so only an actual existing prefix is
normalized; otherwise the prefix is prepended. Same fix applies to the .NET
prefix (e.g. ".NETStandard bump").

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/.github/skills/pull-requests

@@ -0,0 +1 @@
+../../../.github/skills/pull-requests