Host checkouts on a case-insensitive host merge paths (staging clone, rebase worktree) #179

Found while fixing #162.

Problem

createTaskWorkspace (runner/workspace.ts) checks out the task head into a host folder, <runner.root>/<owner>/attempts/<attemptId> (git/clone.ts, clone --no-checkout then checkout --detach). The seeder then copies that folder into the tmpfs work volume (agents/container/storage.ts). The work volume is case-sensitive and compares names as bytes. The host folder may not be: macOS APFS folds case and Unicode normalization by default.

When the head has two paths that differ only by case (or only by normalization), the host checkout keeps one file. The copy to /work then has one file, while the index has two entries.

Reproduced on macOS 26 (APFS) with the same clone and checkout commands:

# tree has README.md ("upper") and readme.md ("lower")
$ ls
readme.md
$ git status --short
 M README.md

So the agent starts from a tree that does not match the head, and change inspection sees an edit nobody made. #162 now checks pathIdentity against the work volume, so the plan keys are correct for /work. That check does not cover this earlier step.

Acceptance

• A head with paths that collide on the host filesystem reaches /work with every entry intact, or the attempt is refused before launch with a message that names the colliding paths.
• A test with two paths that differ only by case fails today on a case-insensitive host.
• Record the choice in docs/implementation/runner-lifecycle.md.