fix(file): do not collapse missing path components onto ancestor files

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Bug and impact

With sandbox workspace_only enabled (the documented default), write_file on a nested path that does not exist yet could silently destroy an existing workspace file:

write_file path=$workspace/important.md/nested.txt content=PWNED
# important.md is a regular file → truncated to "PWNED"

write_file path=$workspace/missing_dir/notes.md content=PWNED
# notes.md already exists at the workspace root → overwritten with "PWNED"

The same ancestor collapse made read_file on secret.env/x return the contents of secret.env.

Agent-generated nested paths (or a typo that treats a file as a directory) are a realistic trigger. The tool returned {"status":"ok"} while clobbering the wrong file.

Root cause

path_within_workspace walks dirname until an existing ancestor realpaths, which is correct as a membership check. file_write then used that ancestor as the fopen target: if it was a regular file, it opened it with "w"; otherwise it appended only the original basename. Missing intermediate directories therefore collapsed onto a parent file with the same name.

Fix

Resolve the intended write path separately:

• existing file → write that canonical path
• new file → require the immediate parent to exist as a directory under the workspace, then write parent/basename

read_file / list_dir now realpath the requested path itself instead of opening the ancestor.

Validation

• make CI=true test_file && ./build/test_file — 34 tests, 0 failed
• New cases: write/read through a file used as a directory must fail and preserve important.md; write to missing_dir/notes.md must fail and preserve workspace notes.md