Fix caching gaps and reduce Pathname overhead

[pharris2411]

Summary

• Cache nil results for unowned files: TeamFinder.for_file previously skipped caching when RustCodeOwners.for_file returned nil, causing repeated calls to the Rust layer for every lookup of an unowned file. Now caches nil so subsequent lookups are instant.
• Memoize Pathname.pwd: The working directory doesn't change during a process, but Pathname.pwd was called on every invocation, issuing a syscall each time. Now memoized in FilePathFinder.pwd.
• Replace Pathname#relative_path_from with string prefix strip: For files under the project root (the common case), we use String#delete_prefix instead of Pathname.new(path).relative_path_from(pwd), avoiding costly Pathname allocations, cleanpath, and cleanpath_aggressive chains. Falls back to Pathname for paths outside the project root.

Context

Profiling CodeOwnership.for_file in a large Rails app (Gusto/zenpayroll) showed:

• 59% of time spent in Pathname.pwd syscalls
• Most remaining time in Pathname#relative_path_from → cleanpath → File.basename chains
• Unowned files were never cached, hitting the Rust layer on every call

Results

This is testing with getting 100 instances of code ownership for an unowned job.

Before, this took about 104 ms

After, this takes 1.41 ms

Test plan

• Updated existing spec to verify nil results from Rust are now cached
• CI passes

🤖 Generated with Claude Code