fix: correct package-manager detection and harden the CLI

[arzafran]

What this does

Omnes advertised monorepo support — "run it from any sub-package and it finds the root lockfile" — but that never worked. The directory walk had an off-by-one bug that made it check only the directory you started in, so running omnes install from packages/api silently fell back to npm even when the root had a pnpm-lock.yaml. This fixes the walk so detection works from anywhere in the tree, and adds support for the Corepack packageManager field that modern repos pin.

It also cleans up a handful of smaller correctness gaps and gives the project tests + CI so these don't regress.

Summary

• Detection walks up to the filesystem root (detectPackageManager). The old root = dirname(startDir) sentinel exited the loop after one iteration. Verified from a subdirectory end-to-end.
• Corepack packageManager field is honored and takes precedence over a lockfile in the same directory; closest directory wins.
• Lockfile priority now matches the README (yarn before npm).
• --version reads from package.json instead of a hardcoded string that had drifted to 0.1.0.
• --help / --version print to stdout (were on stderr, so --version | cat produced nothing).
• Missing package manager exits 127 as the API table documents (was 1).
• Using: line shows the resolved command (Using bun: bun run dev).
• Refactor: pure logic extracted to src/lib.ts; src/omnes.ts is a thin bin.
• Tooling: Biome, typecheck/test/lint/format scripts, GitHub Actions CI, .tldr/ gitignored.
• Release: version bumped to 0.1.3.

Test Plan

• bun run typecheck — clean
• bun run lint (Biome) — clean
• bun run test — 13/13 pass (traversal, Corepack, priority, npm arg transform)
• bun run build — succeeds, tests excluded from dist/
• bun install --frozen-lockfile — no changes (CI-safe)
• Manual: --version → omnes v0.1.3 on stdout; detection from src/ resolves bun via root bun.lock