ci: consolidate structural PR checks into one gate

[ryjones]

Aggregate the structural, PR-metadata checks into a single required status context so branch protection can require one rollup instead of each check individually. The per-package CI check leaves are intentionally left as individual contexts (useful signal) and are not touched here.

Consolidate the two structural PR-lint workflows into one pr-structural-checks.yaml ("PR Structural Checks") holding all four jobs:

• Commit Lint
• Validate PR title
• Validate PR title match with latest commit message
• Validate PR Type

plus a PR Structural Checks Success rollup that needs all of them. It uses if: always() and fails only when a dependency result is failure or cancelled, so a skipped job is treated as passing. This replaces the former commitlint-pull-request.yaml and semantic-pull-request.yaml.

External app checks (DCO, dpulls, mergefreeze) emit their own status contexts and cannot be pulled into a workflow needs graph, so they stay standalone.

Follow-up (manual, after merge): require PR Structural Checks Success in place of the individual Commit Lint / Validate PR * contexts.

Pull Request Requirements

• Rebased onto upstream/main branch and squashed into single commit to help maintainers review it more efficiently and to avoid spaghetti git commit graphs that obfuscate which commit did exactly what change, when, and why.
• Have git sign-off at the end of the commit message (Signed-off-by: Name <email>) to certify the Developer Certificate of Origin (DCO). Use the -s flag with git commit. See signing commits for more information. AI agents must not add Signed-off-by tags — only the human submitter may certify the DCO (see AI Guidelines §7).
• Follow the Conventional Commits specification for commit linting.
• If AI tools were used, include an Assisted-by tag in the commit message per the AI Guidelines §2.2 (e.g., Assisted-by: GitHub-Copilot:claude-opus-4). All AI-generated code must be human-reviewed before submission.
• Read and followed the Pull Request Guidelines.

Note: PRs that do not satisfy the requirements above will fail the
DCO checker and/or the commit lint CI checks and cannot be merged.

Character Limit

• Pull Request Title and Commit Subject must not exceed 72 characters (including spaces and special characters).
• Commit Message per line must not exceed 80 characters (including spaces and special characters).

A Must Read for Beginners
For rebasing and squashing, here's a must read guide for beginners.

[RafaelAPB]

I think each package individual check leafs are useful and should remain as is; however, we could aggregate structural ones (DCO, PR title, PR body checks, etc) into one

[ryjones]

@RafaelAPB If this is how you intended I can mark ready for review whenever