Migrate package manager from npm to pnpm

## Summary

Port this project from npm to pnpm for faster installs, stricter dependency resolution, and disk savings via the content-addressable store.

## Scope

- [ ] Replace `package-lock.json` with `pnpm-lock.yaml` (run `pnpm import` then `pnpm install` to preserve the resolved tree)
- [ ] Add a `packageManager` field to `package.json` (e.g. `"packageManager": "pnpm@"`) to pin the tool
- [ ] Update any `npm install` / `npm run` references in scripts, README/docs, and contributor instructions
- [ ] Update CI workflows to use pnpm (setup-pnpm / corepack, cache key, install command)
- [ ] Update any Dockerfile / release tooling that invokes npm
- [ ] Remove `package-lock.json` once `pnpm-lock.yaml` is committed

## Notes

Verify the published package contents are unchanged (`pnpm pack` vs prior `npm pack`) since this is an MCP server that gets installed/run by others.

Start by inspecting package.json, the existing package-lock.json, CI workflow files, Dockerfiles, release tooling, and documentation for npm references. Run pnpm import and compare the resulting dependency tree and package contents with the prior npm pack output. Done means pnpm-lock.yaml and the pinned packageManager field are committed, npm references are updated, and CI, Docker, release tooling, and packaging remain functional.

Written by the indexing model from the issue text.