cliExists() misses pnpm, Volta, fnm, asdf, and proto global bin dirs

[Railly]

Context

Issue #20 fixed package-manager discovery for skillkit's binary lookup in src/skillkit.ts, but the general-purpose cliExists() in src/tool-configs.ts still walks a shorter hardcoded dir list, so tools installed via pnpm/Volta/fnm/asdf/proto are not detected.

Task

Extract the discovery dir list from findSkillkitBin() into a shared helper and use it in cliExists().

Acceptance criteria

cliExists() finds a CLI installed via each of those package managers; unit test with mocked existsSync added.