feat(sdk): add publish manifest generation and source generators

[CalvinAllen]

Summary

• Add publish.manifest.json generation from VSIX manifest values (publisher, categories, repo)
• Add source generators for VsixInfo and VSCT constants
• Add README template to dotnet new vsix template
• Add Bluesky and LinkedIn notifications to release workflow
• Add contributors workflow
• Update README with publish manifest documentation

Changes

Publish Manifest Generation

Automatically generates publish.manifest.json during build:

• publisher from Identity/@Publisher
• categories from Tags element
• repo from MoreInfo element

Source Generators

• VsixInfoGenerator - generates compile-time constants from VSIX manifest
• VsctGuidsGenerator - generates GUIDs and IDs from .vsct files

Workflows

• Added Bluesky and LinkedIn social notifications to release workflow
• Added contributors workflow to auto-update README

Test plan

• Build sample extension and verify publish.manifest.json is generated
• Verify source generators produce correct output
• Test dotnet new vsix template includes README