chore(chloggen): branch-name default, optional auto-filled PR, type/docs skip

[zhxiaogg]

What this PR does:

Quality-of-life improvements to the new chloggen-based changelog tooling.

• make chlog-new defaults the entry filename to the current branch. A bare make chlog-new previously failed (the tool requires --filename but the Makefile didn't pass it); it now works as the docs describe. Override with FILENAME=.... Defaulting is refused on main/master/detached HEAD with a clear message.
• issues is now optional. When left blank, the PR number is backfilled from the commit that added the entry file (Tempo squash-merges as ... (#1234)). Resolution happens in Go before the summary is rendered. On a release branch the file is added by the backport PR, so the backport PR number is used; set issues explicitly to pin a specific number.
• Strict resolution. Both chlog-update and chlog-preview (--dry) fail if any entry's PR cannot be determined, listing every unresolved entry in one error — they never render or write an entry without a link. (Earlier drafts left preview lenient; it is now strict in both modes. Set issues explicitly when no PR exists, e.g. a direct-to-main commit.)
• PRs labeled type/docs skip the required changelog check (alongside Skip Changelog, dependencies, and chore: titles).
• New Tools Unit Tests workflow runs go test ./... for the separate tools module (tools/go.mod) on every PR, so chloggen's tests run in CI (the main unit-test jobs only cover the root module). Runs on every PR so it can be marked a required check without blocking unrelated PRs.
• Docs/comments clarified in TEMPLATE.yaml, .chloggen/README.md, and CONTRIBUTING.md: issues holds the PR number (an issue number also works) and can be left blank.

Which issue(s) this PR fixes:

Checklist

• Tests updated (tools/chloggen suite green; added TestBackfillIssues and update fail-path tests)
• Documentation added
• Changelog entry — N/A, tooling-only chore: change

[Copilot]

Pull request overview

This PR improves the repository’s chloggen-based changelog workflow by (a) making make chlog-new usable without extra flags, (b) allowing changelog issues to be omitted and backfilled from git history during chlog-update, and (c) updating CI/docs/templates to match the new intended behavior (including skipping changelog enforcement for type/docs PRs).

Changes:

• Add best-effort PR-number backfill for empty issues fields using the commit that added the entry file, and fail chlog-update/chlog-preview when unresolved.
• Improve make chlog-new to default the entry filename to the current git branch (with guardrails for main/master/detached HEAD).
• Update docs/templates/CI: clarify issues semantics, skip changelog check for type/docs, and add a tools-module unit test workflow.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tools/chloggen/internal/chlog/entry.go	Makes issues optional in validation; adds git-based PR backfill + missing-issues detection; tracks entry source path.
tools/chloggen/internal/chlog/entry_test.go	Updates validation expectations and adds BackfillIssues unit tests; ensures test helpers populate entry.path.
tools/chloggen/cmd/validate_test.go	Updates validate behavior tests to treat empty issues as valid.
tools/chloggen/cmd/update.go	Performs issues backfill before rendering and errors out if unresolved (including in --dry).
tools/chloggen/cmd/update_test.go	Adds tests ensuring update/dry-run fail when PR can’t be determined and that errors list all unresolved entries.
Makefile	Defaults chlog-new filename to current branch and adds a safety check for non-derivable branch states.
CONTRIBUTING.md	Updates contributor guidance for chloggen usage, including optional issues and new filename behavior.
.github/workflows/tools-tests.yml	Adds a dedicated workflow to run unit tests for the tools/ Go module.
.github/workflows/changelog.yml	Extends changelog-check skip conditions to include type/docs label.
.github/pull_request_template.md	Updates checklist guidance to mention overriding the default chloggen filename.
.chloggen/TEMPLATE.yaml	Clarifies that issues is optional and can be auto-filled at release time.
.chloggen/README.md	Documents optional issues behavior, backfill semantics, and failure modes for update/preview.

Comments suppressed due to low confidence (1)

CONTRIBUTING.md:451

• MEDIUM: This section recommends running make chlog-preview before pushing while also documenting that issues can be left blank. With the current chlog-preview implementation (chloggen update --dry), entries with blank issues will fail to preview until after merge/backfill (or until issues is set explicitly). It’d help to call out this constraint here so contributors don’t hit a confusing failure.

Validate and preview before pushing:

```bash
make chlog-validate
make chlog-preview

</details>

[zalegrala]

Looks good to me.