Update outdated development and release instructions in CONTRIBUTING.md and README #83

Problem / Use case

CONTRIBUTING.md contains outdated development and release instructions that contradict the actual setup:

• It says npm run build "Compile[s] TypeScript and bundle[s] with esbuild". npm run build only runs tsc. Bundling is npm run bundle, and the bundle must not be committed manually.
• It says "When you bump the version, update all three together". Versions are bumped by release-please in the release PR, so contributors should not bump versions themselves.
• It says CI runs npm run build && npm test. CI now runs on Node 24, and the separate bundle-compat job smoke-tests the bundle on Node 20.0.0 (ci: run on Node 24 and smoke-test the bundle on Node 20 #78).

The README "Releasing" section is also incomplete. It does not mention that the Release workflow regenerates and commits plugin/scripts/bundle.js on the release PR, or that merging the release PR publishes @deploygate/mcp to npm.

Proposed solution

• CONTRIBUTING.md:
  • Fix the development commands to match package.json and the README "Development" section (build = tsc only; bundle regenerates the bundle locally and is never committed).
  • Replace the version-bump instruction: release-please manages versions, and the version-consistency tests exist to catch manual edits.
  • Update the CI description: Node 24, plus the bundle-compat job on Node 20.0.0.
  • Point to the README "Releasing" section instead of duplicating it.
• README "Releasing" → "How a release happens":
  • Add the step where the Release workflow regenerates and commits plugin/scripts/bundle.js on the release PR.
  • Add the npm publish of @deploygate/mcp after the tag is created.
  • Note that plugin users receive the release as soon as the release PR is merged into main.

Additional context

Found while reviewing the release process after 1.5.2 (#80). #82 (retrying a failed npm publish) will add its own README subsection.