Fix spelling typos across codebase

[luojiyin1987]

Fixes #274

This PR fixes multiple spelling typos found in the codebase.

Code Constants

• Fixed `NOTIFICAITON` → `NOTIFICATION` in:
  • `src/openagents/config/globals.py`
  • `src/openagents/core/network.py`

Documentation

• Fixed typos in:
  • `README.md`: "havn" → "haven"
  • `docs/getting-started/quick-start-guide.mdx`: "Afer" → "After", "importan" → "important", "differen" → "different"
  • `docs/tutorials/demo-grammar-check-forum.mdx`: "utlities" → "utilities", "becuase" → "because"

Changelogs

• Fixed typos in:
  • `changelogs/blogs/2025-11-27-demo-showcase.md`
  • `changelogs/docs/2025-11-27-demo-showcase.md`
  • `changelogs/docs/2025-12-20-grammar-forum-bedrock.md`

Studio UI Components

• Fixed typos in sheet.tsx and menubar.tsx

Notes

The following were intentionally NOT changed:

• `network_discovey.py` - filename typo (requires file rename)
• `desitnation_id` - attribute name (API breaking change)
• Grammar checker demo files - intentional typos for demonstration

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