chore(subplat): Add SubPlat skills for fetching Jira tickets and drafting PRs

[david1alvarez]

Because:

• There are several very useful fxa-specific skills, and subplat as of yet does not have any

This commit:

• Adds in two new skills: subplat-jira-fetch (requires locally authenticating the Atlassian MCP server) and subplat-pr-open

Closes #N/A

Checklist

Put an x in the boxes that apply

• My commit is GPG signed.
• If applicable, I have modified or added tests which pass locally.
• I have added necessary documentation (if appropriate).
• I have verified that my changes render correctly in RTL (if appropriate).
• I have manually reviewed all AI generated code.