fix(opencode): prevent literal nul file creation on posix

[cyberprophet]

Issue for this PR

Closes #11403

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This fixes repeated creation of a literal nul file in project directories during shell tool usage.

Changes made:

• added a small normalization utility that rewrites redirection targets nul/NUL to /dev/null on non-Windows platforms
• applied that normalization in both shell execution paths (bash tool and session shell runner)
• changed child stdin from ignore to pipe and explicitly closed it to avoid null-device handling side effects
• added focused tests for redirection normalization behavior

Why this works:

• on POSIX systems, nul is a normal filename, not a device; rewriting redirections prevents accidental file creation
• explicit stdin close avoids relying on runtime-specific ignore behavior

How did you verify your code works?

• Added unit tests in packages/opencode/test/util/redirection.test.ts
• CI checks on this PR show typecheck and unit tests passing on linux/windows
• Local verification command output in my environment had dependency/tooling gaps (glob, tsgo, @opentui/solid/preload), noted transparently in prior description

Screenshots / recordings

N/A (non-UI change)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[cyberprophet]

Updated the PR description to match the required template and kept the issue linkage (). Please re-check compliance constraints.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[cyberprophet]

Updated the PR description to match the required template and kept the issue linkage (Closes #11403). Please re-check compliance constraints.