fix(tool-sandbox): skip missing fs_read/fs_write dirs instead of erroring

[kipz]

Linked Issue

Closes #1252

Summary

fs_read_file entries in add_policy_fs already silently skip missing paths via add_optional_read_file. fs_read and fs_write directory entries used
FsCapability::new_dir(...)? which propagated PathNotFound as a hard error, preventing the tool-sandbox from starting when a profile grants a directory that
doesn't exist on that machine.

Add add_optional_dir with the same warn-and-skip pattern and use it for both fs_read and fs_write in add_policy_fs on macOS and Linux.

Test Plan

Existing nono-cli test suite passes. Manual verification: profile with a non-existent fs_read directory no longer errors on tool-sandbox startup.

Checklist

• An issue exists and is linked above
• All commits are signed-off, using DCO
• All new code follows the project's coding standards (CLAUDE.md) and is covered by tests
• Public-facing changes are paired with documentation updates
• Release note has been added to CHANGELOG.md if needed

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+28
Lines removed	-4
Total changed	32
Classification	Small (< 50 lines)

Affected crates

• crates/nono-cli — CLI changes. Verify argument parsing, flag documentation, and UX behaviour across supported platforms.

Blast radius — Moderate

This PR touches: source code,documentation

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[gemini-code-assist]

Code Review

This pull request updates the tool-sandbox platform implementations for Linux and macOS to skip missing fs_read and fs_write directories during startup instead of throwing an error. This is implemented via a new helper function add_optional_dir which handles NonoError::PathNotFound gracefully, matching the existing behavior for files. There are no review comments to evaluate, and the changes look correct and consistent.

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