fix(agent): resolve oauth cache home cross-platform

[kalvinnchau]

Summary

• resolve the OAuth cache home directory with the repository's platform-aware dirs convention
• preserve the existing .config/buzz-agent/oauth cache layout on macOS and Linux
• make the cache-path regression assertion portable across path separators

Problem

buzz-agent read only $HOME when constructing the OAuth token cache path. Packaged Windows processes do not guarantee that Unix variable, so OAuth source construction failed with oauth cache: $HOME not set even though Windows had a valid user profile.

Validation

Independent reviewers validated exact commit 836d820483b141b7291170cb33535ac7cb49b2eb on Windows/MSVC with HOME unset and USERPROFILE present:

• cargo +1.94.1 clippy -p buzz-agent --all-targets --locked -- -D warnings
• cargo +1.94.1 fmt --all -- --check
• git diff --check f53bbd1..836d820
• auth:: tests: 11/11 passed with HOME unset
• full package lib target: 396 passed / 2 failed; identical-base controls classified both as pre-existing Windows failures

The changed regression fails on the base with $HOME not set and passes on this branch. dirs 6.0.0 was already locked by other workspace crates; the lockfile change adds only the buzz-agent dependency edge.