[inureyes]

HOME-sensitive unit tests now share the writer's lock, including indirect reads through SSH configuration resolution, authentication defaults, session policies, and KnownHostsCommand contexts. Existing HOME writers keep their single guard; the helper documents the non-reentrant locking rule.

The plain-key fallback test constructs a KnownHostsCommand context that reads HOME; its script, output, and log use a unique temporary directory. Its original failure did not reproduce (0/1,000 before and after), so this does not claim to establish that flake's cause. Assertion diagnostics now include invocation logs and command output, and tools/test-home-races.py preserves stdout, stderr, backtraces, timeout failures, and a summary for every run.

Validation:

• Before: 36 failed invocations / 1,000, with 14 tests per invocation; captured HOME mismatches included /home/user/.ssh/other versus /home/inureyes/.ssh/other.
• After: 0 failed invocations / 1,000 using the same 14 tests, eight concurrent processes and eight test threads each. The selection combines both reported readers, other known-host tests, the fallback test, and the HOME-mutating tilde test.
• 254 available guarded tests passed in batches of at most 30, each also running the HOME writer; formatting and lib/test clippy passed.

Audit exclusions: app tests run in a separate binary without HOME mutations; keygen tests never call the HOME-reading entrypoint; config-cache tests load explicit paths; the historical interactive_unit_test.rs is not compiled, while its current replacement already locks HOME. Production behavior is unchanged.

Closes #335