[matt-edmondson]

Fixes #162

What changed

• ProcessDirectory calls a new EnsureWritable right after Directory.CreateDirectory. It opens a dot-named, uniquely named file in the output folder with FileMode.CreateNew and FileOptions.DeleteOnClose. When the folder is read-only, that throws UnauthorizedAccessException or IOException before any icon is decoded. The existing catch in Execute then reports "Argument validation failed" and exits 1, which is the same result as an output folder that can't be created.
• The probe name starts with a dot, so if a crash ever leaves one behind, a later run that uses the folder as input skips it as metadata.
• Updated the exit-code docs in the README and CLAUDE.md.

Tests

• AnExistingReadOnlyOutputFolderExitsWithInvalidArgumentsAndWritesNothing: the output folder is mode 555 and already exists. The test expects exit 1 and nothing written, the probe included. It reports Inconclusive on Windows, and also when the process can still write to the folder (root, or a file system that ignores mode bits).
• LeavesNoWriteProbeBehindInAWritableOutputFolder: on a normal run, the output folder holds only icon.png.

I proved the read-only test outside root by running the test executable under setpriv --inh-caps=-all --bounding-set=-all, which drops CAP_DAC_OVERRIDE:

• With the fix: 90 passed, 0 failed, 0 skipped.
• With the EnsureWritable call removed: the read-only test fails, because Execute returned 2 instead of 1.

Under plain root, the read-only test reports Inconclusive as designed, and the other 89 pass. CI runners are not root, so the test runs there.

I ran the suite locally with -p:NuGetAudit=false. Restore on main currently fails with NU1903 on SixLabors.ImageSharp 3.1.12, which #145 bumps.

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[matt-edmondson]

This PR's CI is red because of main, not this change, and the fix is a maintainer decision.

• The Test jobs fail at restore, before any test runs, with NU1903: Warning As Error: Package 'SixLabors.ImageSharp' 3.1.12 has a known high severity vulnerability (GHSA-v76p-62qx-wwq2). main has the same failure.
• I ported Bump SixLabors.ImageSharp from 3.1.12 to 3.2.0 #145's bump to 3.2.0, which is the only release with the fix. With it, CI then failed with No Six Labors license found. Set $(SixLaborsLicenseKey)…. ImageSharp 3.2.0's build targets fail any CI build that has no license key. There is no patched 3.1.x release.
• I reverted the bump (0991902), so this PR carries only the fix for An existing but read-only output folder exits 2 and fails every file one by one, while an uncreatable one exits 1 #162. Bump SixLabors.ImageSharp from 3.1.12 to 3.2.0 #145 will hit the same license error.

To unblock main and this PR, choose one of these:

• Add a Six Labors license key as SixLaborsLicenseKey.
• Suppress this advisory with NuGetAuditSuppress while you decide.
• Move off ImageSharp.

The tests for this change pass locally on 3.1.12 with the audit turned off. With file-permission overrides dropped (non-root), 90 passed and 0 failed.

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