feat: integrate credential manager

[michaelmalave]

Summary

Replaced direct netrc parsing with the heroku-credentials-manager from @heroku-cli/command@13.0.0-beta.1 for token retrieval.

Type of Change

• fix: Bug fix or issue (patch semvar update)
• feat: Introduces a new feature to the codebase (minor semvar update)
• perf: Performance improvement
• docs: Documentation only changes
• tests: Adding missing tests or correcting existing tests
• chore: Code cleanup tasks, dependency updates, or other changes

Verification

# Build and run tests
npm run build
npm test

# Manual verification (requires heroku login)
./bin/run devcenter:pull slowdb 
./bin/run devcenter:push slowdb

# Test with HEROKU_API_KEY env var
HEROKU_API_KEY=your-token ./bin/run devcenter:pull slowdb

All 72 tests pass with 80.23% coverage (meets 80% threshold).

Additional Context

• Updated @heroku-cli/command from ^12.2.0 to ^13.0.0-beta.1
• Removed direct netrc-parser dependency (now handled by credential manager)
• Token retrieval precedence: HEROKU_API_KEY env var → native keychain → netrc fallback
• getHerokuApiToken() is now async; commands updated to use await
• Basic auth encoding preserved for Dev Center API compatibility
• Added esmock as dev dependency for ESM module mocking in tests
• Deleted unused test helpers (netrc-path.ts, test-home-env.ts)
• Added additional paths.ts tests to maintain coverage threshold

Related Issue

Closes #W-22034106