feat!: migrate Dev Center CLI to TypeScript Heroku oclif plugin

[michaelmalave]

Summary

Replaces the legacy Ruby gem CLI with the npm Heroku plugin @heroku-cli/plugin-devcenter (oclif v4, ESM, TypeScript). Implements devcenter:open, devcenter:preview, devcenter:pull, and devcenter:push with a TypeScript client, preview server, and shared article resolution.
Authentication uses the Heroku CLI stack (this.heroku.getAuth() / @heroku-cli/command) instead of standalone netrc parsing.

• Removes the Ruby gem layout (Gemfile, lib/, bin/devcenter, etc.),
• Adds oclif bin/run tooling, CI (including coverage via c8), Dependabot/PR template updates,
• Publishes as v2.0.0 with README/package metadata aligned to the released package.

Type of Change

• fix: Bug fix or issue (patch semvar update)
• feat!: Introduces a new feature to the codebase and removes the Ruby gem distribution (breaking change; major semver)
• perf: Performance improvement
• docs: Documentation only changes (README/command docs generated/updated as part of the migration; not docs-only PR overall)
• tests: Adding missing tests or correcting existing tests
• chore: Code cleanup tasks, dependency updates, or other changes (CI, Dependabot, repo hygiene)

If you want a single primary checkbox, use feat! and note the rest in Additional Context.

Verification

npm install
npm run build

Must be logged in with heroku login

heroku devcenter:pull slowdb
heroku devcenter:push slowdb
heroku devcenter:open slowdb
heroku devcenter:preview slowdb

CI should run the same build/test/lint path (pretest runs build; posttest runs lint).

Additional Context

• User-facing breaking change: the Ruby gem and bin/devcenter workflow are removed; users install heroku plugins:install @heroku-cli/plugin-devcenter and run heroku devcenter:* (Node ≥22).
• Auth: Heroku session/token via CLI (getAuth()), shared article-resolve for pull/open, client hardening (e.g. getJson invalid/empty body handling, pull debug logging without dumping full article JSON).
• Repo: package rename to @heroku-cli/plugin-devcenter, .c8rc / 80% coverage expectations, .github/workflows, Dependabot, PR template; latest commit on branch aligns package.json / lockfile / README to v2.0.0 after publish.

Related Issue

Closes W-20888419

[tlowrimore-heroku]

LGTM!