fix(tools): reject unresolved parent traversal (C-03)

[PierrunoYT]

Fixes #13.

Problem

resolve_in_workspace canonicalized the deepest existing ancestor, then appended unresolved path components without lexical normalization. A path such as target/missing/../../../outside.txt could therefore pass starts_with(cwd) and later escape when a file tool created parent directories.

Resolution

• Reject every ParentDir component before canonicalizing or rebuilding the path.
• Preserve existing support for valid files and new nested workspace paths.
• Cover the vulnerable nonexistent-prefix form in the shared workspace helper.
• Add file-write and file-edit regression tests.
• Add a Windows backslash traversal regression test.

Validation

• Focused workspace tests: 6 passed.
• Focused file tool traversal tests: 2 passed.
• cargo test --locked --all-targets -- --test-threads=1: 227 passed.
• The default parallel suite reproduced the existing unrelated M-08 environment-test race in notify::tests::enabled_defaults_true_without_env; that test passed independently.

Audit finding: C-03.

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📥 Commits

Reviewing files that changed from the base of the PR and between 4208cd2 and 16a5eb4.

📒 Files selected for processing (5)

• src/tools/file_edit.rs
• src/tools/file_history.rs
• src/tools/file_undo.rs
• src/tools/file_write.rs
• src/tools/workspace.rs

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[PierrunoYT]

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[euxaristia]

LGTM! Approved after audit.