Isolate tmp-dependent tests from ambient git

[starr-openai]

Why

The remote full-ci path has been picking up host-dependent git state from temp directories, which makes unrelated tests fail when /tmp or another ancestor looks like a repository. This PR removes that ambient dependency from the first layer of the CI reliability stack.

What changed

• isolate project-root-sensitive test fixtures from ambient git metadata
• make the TUI chatwidget test project path unique per test run, while normalizing snapshots back to /tmp/project
• keep repo-root-sensitive core-skills and secrets tests from depending on whatever git markers happen to exist above their temp paths

Stack

• PR #22199 Isolate tmp-dependent tests from ambient git
• PR #22201 Use remote fs for turn diff repo root
• PR #22200 Emit unified exec end on startup failure
• PR #22389 Stabilize remote routing e2e tests

Validation

• Current-head CI is green.
• No local Codex test run; relying on CI for this stack.