EFFECT-TS-002: Convert fileCheckers.ts to @effect/platform FileSystem

[timbrinded]

Parent Epic

#555

Milestone

1 — Foundation Services

Summary

Convert fileCheckers.ts from callback-based file I/O to @effect/platform FileSystem with typed errors.

Problem

• checkExists() mixes file I/O with architecture validation
• getBinaryArchitecture() uses callback-based file reading for binary header inspection
• downloadBinsIfMissing() interleaves interactive prompts, file I/O, and subprocess invocation with untyped error handling
• @effect/platform FileSystem is already in the dependency tree but unused here

Scope

• Convert checkExists to use FileSystem.exists + FileSystem.access
• Convert getBinaryArchitecture to use FileSystem.readFileString or FileSystem.read
• Create FileCheckError, BinaryArchitectureError tagged error types
• Convert downloadBinsIfMissing to Effect.gen with proper error channel

Files

• Modify: src/cli/internal/fileCheckers.ts

Depends On

Nothing — can start immediately.

Size

S (~150 LOC removed, ~100 LOC added)

[timbrinded]

Closed by PR #567 — fileCheckers.ts has been migrated to idiomatic Effect.gen pipelines with typed TaggedErrors and DI services.