Promote InnerSource Hackathon to Level 2 (Structured)

[rrrutledge]

Summary

• Moves the InnerSource Hackathon pattern from 1-initial to 2-structured for publication in the InnerSource Patterns book.
• Enhances the Solution section with structured guidance on preparation, participation, and event format.
• Adds a Related Patterns section linking to 5 related patterns.
• Adds the pattern to the InnerSource Program Mind Map under Begin > Program Setup.
• Rewrites the patlet to follow the two-sentence problem/solution format.
• Minor grammar and style fixes throughout.

Note: This PR depends on #899 which adds The Aerospace Corporation as a Known Instance. That PR should be reviewed and merged first so the diff here is limited to structural/promotion changes only.

Level 2 Requirements Checklist

• Validated by at least 1 known instance (has 2: IKEA and The Aerospace Corporation)
• Uses the format described in the Pattern Template
• Follows the Pattern Style Guide
• Title is memorable, concise, and descriptive
• Links to related patterns of this level or higher
• Links to outside resources reference trusted resources
• Added to at least one phase of the InnerSource Program Mind Map
• Spelling & Style checks pass (to be verified by CI)
• Markdownlint checks pass (to be verified by CI)

Test plan

• Verify CI checks pass (markdownlint, spelling, link checker)
• Review structural changes to Solution section for accuracy and completeness
• Confirm mind map entry renders correctly
• Verify related pattern links resolve to existing Level 2 patterns

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