docs: improve Partuno discoverability and registry metadata

[JPMarhefka]

Summary

• Reposition the README around DigiKey and Mouser MCP workflows while keeping Partuno as the product name.
• Add practical search-focused guides for DigiKey MCP, Mouser MCP, and electronics BOM MCP use cases.
• Add PyPI metadata, package keywords, launcher documentation, and the official MCP Registry server.json.
• Update API and MCP descriptions to reflect provider-neutral, local-first component sourcing.

Validation

• 159 tests passed
• 4 tests skipped
• server.json validated by mcp-publisher validate
• uv build --no-sources completed successfully
• PyPI source distribution contains the MCP ownership marker

After this PR is merged, publish Partuno 4.0.0 to PyPI, then publish the matching server.json to the MCP Registry.