[codex] Clarify bun lockfile detection errors

[aouxwoux]

Fixes #56.

What changed

• Detect the actual Bun lockfile path, including bun.lockb.
• Add an explicit package manager override through --package-manager and UPDATE_BROWSERSLIST_DB_PM.
• Show a clearer error when a Bun lockfile selects Bun but bun is not available on PATH.
• Document the override and add regression coverage for the Bun error and override path.

Verification

• pnpm unit
• pnpm test:lint
• node cli.js --help
• node cli.js --package-manager exits with the expected missing-value error

Note: pnpm test reaches the same passing unit/lint steps locally, but Windows skips the existing Yarn/Bun probes because they redirect to /dev/null; CI runs on Ubuntu with Bun installed.

[ai]

This part is not very readable. Let’s add getArgValue() helper.

[aouxwoux]

pushed a follow-up for the review notes:

• moved the missing/flag-looking package-manager value check into getArgValue()
• removed the extra isArg(--package-manager) validation branch
• moved lockfile path setup into getLockfile()
• changed auto-detection to keep the first lockfile object instead of calling getLockfile() twice

checks passing locally with npm/pnpm shims:
pnpm unit, pnpm test:lint, git diff --check