docs: add README for the npm example plugin (#15)

[ben-laird]

Closes #15.

examples/plugins/npm/main.ts is already functionally complete: it implements the full op set — info, discover, read-version, write-version, update-dependency, get-dependencies, release, finalize — targets package.json, and finalize runs npm install --package-lock-only so package-lock.json refreshes in the same commit (reported in additionalChangedFiles only when it differs from HEAD). The only gap vs the deno example was a missing README.

This PR adds examples/plugins/npm/README.md, modeled on examples/plugins/deno/README.md: wiring via the run: arm, per-op descriptions (including info, get-dependencies, and finalize, which the deno README predates), and npm-specific adaptation notes (Node rewrite, --package-lock-only, permissions).

Verification

$ dv plugin verify "run:deno run -A .../examples/plugins/npm/main.ts"
  ✓ info  contractVersion=1, 8 ops declared (npm 0.1.0)
  ✓ discover  0 packages returned for glob '*'
  · read-version / write-version / update-dependency / release / get-dependencies  (side-effectful or no-package skips)
  ✓ finalize  no-op run reported 0 additional files
  ✓ bad-input rejects
PASS  4 passed, 0 failed, 5 skipped

(The skipped ops are side-effectful — verify can't safely auto-run them — identical to the deno example's verify profile.)

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