docs: route PR creation by remote platform

[VIVAAN-DHAWAN]

Fixes #1609

Summary

• Add remote host detection before the finishing-a-development-branch skill creates a PR/MR.
• Document the right tool for GitHub, GitLab, Bitbucket, and unknown remotes.
• Add a common mistake entry for using gh on non-GitHub repositories.

Verification

• Confirmed the old skill text lacked GitLab/non-GitHub guidance with: rg -n "glab mr create|git remote get-url origin|non-GitHub" skills/finishing-a-development-branch/SKILL.md
• Confirmed the new guidance is present with the same rg check.
• git diff --check

[YOMXXX]

Triage note for reviewers:

This overlaps directly with #1609 and the follow-up shape discussed after #1613 was closed. The maintainer guidance there was to keep the fix to roughly the one GitHub-specific gh pr create example: a small platform-neutral note like "use the PR/MR tool that matches the remote," rather than expanding the skill into per-platform routing guidance.

A few concrete review points:

• This PR targets main; current contribution flow appears to expect dev.
• The PR body does not use the current PR template sections.
• The diff still adds a multi-platform table plus a Common Mistakes entry, so it is closer to the oversized fix: route finishing PR creation by remote host #1613 shape than the smaller replacement requested by the maintainer.
• The verification is a presence/absence rg check for strings in the diff, not behavior evidence.

Given the existing #1609 discussion, I think the reviewer decision is mostly about whether to ask for a much smaller replacement diff rather than reviewing this exact shape line-by-line.