fix: support stdin-backed file options on Windows

[shaoohh]

Summary

Fix cross-platform support for documented - stdin inputs:

• replace the POSIX-only /dev/stdin path with file descriptor 0
• route mmx text chat --messages-file - through the shared helper
• keep --text-file - and --lyrics-file - working through the same helper on Windows, macOS, and Linux

Root cause

The shared reader and text-chat command hard-coded /dev/stdin, which does not exist on Windows.

Tests

• Adds regression coverage that the stdin marker resolves to file descriptor 0.
• Isolated TypeScript check passes.

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[NianJiuZst]

No blocking findings on the current head. Replacing the POSIX-only /dev/stdin path with file descriptor 0 preserves the documented --*-file - interface and makes it work on Windows, so I recommend merging it.

Verified against current main: focused test, a real piped text chat --messages-file - --dry-run probe, typecheck, and lint (one pre-existing warning). #208 and #229 also pass the combined full suite (402/402).