[ci] fix: accept stacked-PR title prefixes and gate on the PR author

[JoyboyBrian]

What

• Accept an optional leading [N/M] in PR titles, before [BREAKING], and validate that 1 <= N <= M, M >= 2, with no leading zeroes.
• Gate title validation and auto-labeling on the PR author rather than the actor that triggered the workflow run.
• Reconcile title-derived labels whenever a PR is opened, reopened, or its title changes: remove stale type/module/breaking/stack labels, add the labels implied by the latest title, and preserve priority, triage, and other manually managed labels.
• Add the stacked-pr label for [N/M] titles and recognize [BREAKING] after an optional stack prefix.
• Parse only leading module brackets so bracketed text in the description cannot create accidental module labels.
• Document the stacked title format, labels, and gh stack link/submit workflow in CONTRIBUTING.md, the PR template, and the create-PR skill.

Why

The [N/M] prefix used by this repository's stacked PRs did not match the title check, so every stack needed to ignore or work around a failing gate. The convention was also absent from the contributor docs and PR template.

The title check used github.actor, which identifies whoever triggered the current run rather than who opened the PR. A bot touching a human-authored PR could therefore skip validation and make an invalid title appear green.

Auto-labeling previously only added labels. After a title changed, labels derived from the old title remained indefinitely. It also scanned bracketed text anywhere in the title, so description text such as [server] could be mistaken for a module. Reconciliation makes the latest title the source of truth for title-managed labels while leaving unrelated labels untouched.

How to Test

• Parse both workflows, syntax-check the title validator's Bash, and syntax-check the auto-labeler's JavaScript in the async wrapper used by actions/github-script:

  uv run python -c "import yaml; yaml.safe_load(open('.github/workflows/check-pr-title.yml')); yaml.safe_load(open('.github/workflows/auto-label-pr.yml'))"
  uv run python -c "import yaml; print(yaml.safe_load(open('.github/workflows/check-pr-title.yml'))['jobs']['check-title']['steps'][0]['run'])" | bash -n
  uv run python -c "import yaml; s=yaml.safe_load(open('.github/workflows/auto-label-pr.yml'))['jobs']['label']['steps'][0]['with']['script']; print('(async()=>{\\n'+s+'\\n})()')" | node --check

• Execute the title workflow's exact Bash block against standard, stacked, stacked-breaking, zero, leading-zero, reversed, missing-module, and values beyond Bash's fixed-width integer range. Valid cases pass; [0/2], [1/1], [3/2], [01/02], [1/0], [9223372036854775808/2], and [1/2] fix: x fail, while [1/9223372036854775808] passes without overflow.
• Execute the auto-label workflow's exact JavaScript with mocked github, context, and core objects:
  • changing a stacked breaking rollout refactor into a CLI fix removes breaking, refactor, rollout, and stacked-pr, then adds bug and cli;
  • priority: high remains untouched;
  • [ci] fix: document [server] labels adds only bug and ci;
  • invalid stack positions fail without label writes;
  • an already-correct label set performs no writes.
• git diff --check
• gh pr checks 294 --repo Osmosis-AI/osmosis-sdk-python

Checklist

• PR title follows [module] type: description format
• Appropriate labels added
• Workflow YAML, Bash, and JavaScript parse successfully
• Title and label reconciliation cases exercised directly
• Full GitHub CI passes
• Public contributor documentation updated
• No secrets or credentials included