feat: publish Docker image to GHCR via CI

[tallpsmith]

Summary

Automate building and pushing the pmmcp Docker image to GitHub Container Registry (GHCR) so users can run it without cloning the repo.

Acceptance Criteria

• GitHub Actions workflow builds and pushes image to ghcr.io/<org>/pmmcp on push to main and on version tags
• Image is tagged with both latest and the semver tag (e.g. v0.1.0)
• README Installation section updated with working docker pull instructions
• Image runs correctly: podman run -i --rm --name pmmcp ghcr.io/<org>/pmmcp --pmproxy-url http://host:44322

Post-publish cleanup

• Update .mcp.json.example to reference the GHCR image URL instead of the local build (pmmcp)
• Remove the "not yet published" note from .mcp.json.example
• Remove the build-from-source prerequisite from the README "Try It Out Locally" section

Context

The README previously documented this as "Option A (recommended)" but the image was never published. Users currently must build from source.