fix: add platform detection for PR/MR creation (issue #1609)

[ElizioMartins]

Who is submitting this PR?

Field	Value
Your model + version	Nemotron 3 Ultra (via OpenCode)
Harness + version	OpenCode (latest)
All plugins installed	superpowers
Human partner who reviewed this diff	Elizio Martins

What problem are you trying to solve?

Issue #1609: The finishing-a-development-branch skill hardcoded gh pr create for Option 2 (Push and Create PR) without first checking whether the repo is actually hosted on GitHub. On any other platform (GitLab, Bitbucket, self-hosted), gh fails with a confusing error and no PR/MR is created.

What does this PR change?

Adds platform detection before creating PR/MR:

1. Gets git remote get-url origin
2. If URL contains "github.com" → uses gh pr create
3. If URL contains "gitlab" or "gitlab.com" → uses glab mr create
4. Otherwise → echoes manual instructions with fallback URL

Also includes the issue-closing keywords template (Closes #<issue-number>) in both GitHub and GitLab templates.

Is this change appropriate for the core library?

Yes - this affects all users creating PRs/MRs via the skill, regardless of project domain or Git hosting platform.

What alternatives did you consider?

• Auto-detect from multiple remotes (origin only for simplicity)
• Support Bitbucket/Gitea/Gogs (can be added incrementally)
• The fallback to manual URL covers all unsupported platforms

Does this PR contain multiple unrelated changes?

No - single cohesive change adding platform detection to PR/MR creation.

Existing PRs

• I have reviewed all open AND closed PRs for duplicates or prior art
• Related PRs: None found for this specific fix

Environment tested

Harness	Harness version	Model	Model version/ID
OpenCode	latest	Nemotron 3 Ultra	nemotron-3-ultra-free

Evaluation

• Initial prompt: "Add platform detection for PR creation per issue finishing-a-development-branch: hardcodes gh pr create without detecting platform (breaks on GitLab) #1609"
• Eval sessions: 1 (verified platform detection logic is correct)
• Before: gh pr create hardcoded, failed on non-GitHub; After: detects platform, uses correct tool

Rigor

• This change was tested adversarially, not just on the happy path
• I did not modify carefully-tuned content (Red Flags table, rationalizations, "human partner" language)

Human review

• A human has reviewed the COMPLETE proposed diff before submission