Use idiomatic Effect for workspace file reads

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What Changed

• Replaced the node:fs/promises read path in WorkspaceFileSystem.readFile with Effect platform FileSystem APIs (realPath, scoped open, handle stat/read).
• Added granular Schema.TaggedErrorClass causes for workspace read safety failures.
• Converted the workspace filesystem tests to assert style and added coverage for truncation and binary-file rejection.

Why

This keeps workspace file reads inside Effect services end-to-end, avoids direct Node filesystem APIs, and makes read failures more structured and testable while preserving existing read behavior.

Checklist

• This PR is small and focused
• I explained what changed and why
• UI changes not applicable

Verification

• pnpm run test src/workspace/WorkspaceFileSystem.test.ts (from apps/server) — 8 tests passed
• pnpm exec vp check — 0 errors, existing unrelated lint warnings
• pnpm exec vp run typecheck — passed

  

Note

Refactor WorkspaceFileSystem.readFile to use idiomatic Effect APIs with typed error causes

• Replaces node:fs/promises calls with Effect FileSystem APIs (realPath, open, stat) in WorkspaceFileSystem.ts.
• Adds three typed error classes — WorkspaceReadFileResolvedOutsideRootError, WorkspaceReadFileNotFileError, and WorkspaceReadFileBinaryFileError — as structured causes inside WorkspaceFileSystemError.
• Binary files are now explicitly rejected before decoding by scanning the read buffer for a NUL byte.
• Updates tests in WorkspaceFileSystem.test.ts to use @effect/vitest assertions and adds coverage for truncation and binary file rejection.

^{Macroscope summarized 201f654.}