fix(tsconfig): name the OS error when the tsconfig file cannot be read

[colinhacks]

An unreadable tsconfig produced the same line as a missing one, and the verdict under it then told the reader to fix a config that was fine. Seen when a macOS terminal lost TCC access to ~/Documents for a few minutes: every nub <file> in a project there stopped on the tsconfig line with nothing pointing at the OS.

The read diagnostic now carries the OS error, as the parse diagnostics on the same path already do.

$ nub main.ts
Nub: Cannot resolve tsconfig at path: /Users/colinmcd94/tmp/app/tsconfig.json: Permission denied (os error 13)
Error: the project's tsconfig.json could not be read in full (see above).
Fix the config, or use --node to run without Nub's TypeScript features, path aliases included.

Unit test covers the unreadable file on unix and skips under root, which reads through the mode bits. Follow-up to #778.

https://claude.ai/code/session_01CD2jAnXLg1fVRzyRNAYTBr

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Project	Deployment	Actions	Updated
nub	Ready	Preview	Sep 5, 2026 11:31pm UTC

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[pullfrog]

✅ No new issues found.

Reviewed changes Checked the complete one-file patch and its effect on the CLI's tsconfig refusal path.

• OS error context — read_jsonc now preserves the underlying std::io::Error when a discovered tsconfig cannot be read.
• Regression coverage — The Unix permissions test distinguishes the old path-only diagnostic from the new cause-bearing message and safely skips privileged runners.

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