ci: migrate releases to release-please

[tolzhabayev]

Summary

Replaces the do-release.yml / do-release-mcp.yml workflow_dispatch flow with release-please, aligning with the convention used in grafana/plugin-ci-workflows and grafana/plugin-actions. The previous flow is blocked by the org-level "Public repo required PRs" ruleset (see failed run https://github.com/grafana/plugin-validator/actions/runs/25546676728), and bypassing it isn't an option (org-scoped, requires org owner, would weaken the policy across all public Grafana repos).

Supersedes #570 — that PR's bridge fix is replaced by this convention-aligned approach.

What changes

• release-please-config.json + .release-please-manifest.json: two packages, . (plugin-validator) and mcp-package (mcp), release-type: "node", tag-separator: "/", conventional-commits changelog sections matching the reference repos. Root pins bootstrap-sha to the v0.41.0 commit so the first release PR doesn't enumerate all of history.
• .github/workflows/release-please.yml: on push to main (or workflow_dispatch), opens / updates per-package release PRs. Merging a release PR produces the version tag + GitHub Release with the changelog body. Mirrors plugin-ci-workflows/.github/workflows/release-please.yml but drops the rolling major/minor tag step (not needed — this repo's consumers don't pin to floating major versions).
• .github/workflows/release.yml: trigger now matches plugin-validator/v[0-9]*. New Extract release version step exposes RELEASE_VERSION (the bare semver) to GoReleaser.
• .goreleaser.yaml: adds archives.name_template using RELEASE_VERSION. Required because the new tag contains a /, which would otherwise pollute the default {{ .Version }} archive name. Default release.mode: keep-existing is preserved — verified via goreleaser docs: "GoReleaser will not override [the existing release body] with its release notes, unless you configure it to do so (e.g. mode: replace)."
• .github/workflows/release-mcp.yml and .goreleaser.mcp.yaml: unchanged. release-please naturally produces mcp/v* tags that already match the existing trigger and MCP_VERSION extraction.
• package.json binWrapper.urlTemplate: updated to the new download path (releases/download/plugin-validator/v{{version}}/...). Old npm versions ship with the old URL and continue to resolve from existing releases.
• HOW_TO_RELEASE.md: rewritten for the conventional-commit-driven flow.
• Deletes do-release.yml and do-release-mcp.yml.

Tag format change

Root tags change from v0.41.0 → plugin-validator/v0.41.1. MCP keeps its existing mcp/v* format. Existing tags are untouched.

Verified the downstream consumer at grafana/community-pipeline/pkg/lib/getvalidator/get_validator.go is unaffected — it queries /repos/grafana/plugin-validator/releases/latest and filters assets by substring linux_amd64, never pattern-matching on tag names.

Test plan

• Merge this PR.
• Confirm Release Please runs on the merge to main and opens a chore(plugin-validator): release X.Y.Z PR.
• Squash-merge the release PR.
• Confirm the plugin-validator/v<X.Y.Z> tag is created and a GitHub Release exists with the changelog body.
• Confirm Create release and publish triggered, GoReleaser uploaded plugin-validator_<ver>_<os>_<arch>.tar.gz assets, npm view @grafana/plugin-validator@<X.Y.Z> resolves, and Docker tag grafana/plugin-validator-cli:v<X.Y.Z> exists.
• npx -p @grafana/plugin-validator@<X.Y.Z> plugincheck2 --help — verify the binWrapper resolves the new URL path.

[academo]

looks good. let's do a test after this merges