feat: leverage the heroku credential manager for auth

[michaelmalave]

Summary

• Adds @heroku/heroku-credential-manager as the single path for storing, reading, and removing Heroku CLI tokens.
• Login persists credentials via saveAuth; logout keeps server-side revoke (session + OAuth authorizations) and clears local storage via removeAuth with vars.apiHost / vars.httpGitHost (fixes hardcoded netrc hosts for custom HEROKU_HOST).
• API requests attach the Bearer token via await getAuth() (credential manager + HEROKU_API_KEY).
• Command.init() runs await this.heroku.getAuth() so this.heroku.auth is populated before run(), preserving legacy sync checks (e.g. auth:whoami) without updating every plugin.
• Adds src/credential-manager.ts with setCredentialManagerProvider() so tests can inject mocks
• Removes netrc-parser from this package; netrc I/O lives in heroku-credential-manager.
• CI: NODE_AUTH_TOKEN from NPM_TOKEN_HEROKU_CREDENTIAL_MANAGER
• Tests: preload in init, getAuth caching, logout → removeAuth

Type of Change

Breaking Changes (major semver update)

• Add a ! after your change type to denote a change that breaks current behavior

Feature Additions (minor semver update)

• feat: Introduces a new feature to the codebase

Patch Updates (patch semver update)

• fix: Bug fix
• deps: Dependency upgrade
• revert: Revert a previous commit
• chore: Change that does not affect production code
• refactor: Refactoring existing code without changing behavior
• test: Add/update/remove tests

Testing

Checkout this branch for heroku-cli-command.
Open the cli repo to the main branch. In the package.json, update your cli command dependency to:

"@heroku-cli/command": "/path/to/heroku-cli-command",

run npm install && npm run build

Then test with

DEBUG=heroku-credential-manager ./bin/run login
HEROKU_NETRC_WRITE=true DEBUG=heroku-credential-manager ./bin/run whoami
DEBUG=heroku-credential-manager ./bin/run logout

and you should now see heroku-credential-manager logging as its being leveraged.
(note: expect debug lines from the credential manager (e.g. netrc load/save); Keychain usage is not always logged.)

Related Issues

GitHub issue: #[GitHub issue number]
GUS work item: W-20825872

[tlowrimore-heroku]

LGTM!