test(zsh): add interactive E2E harness

[lemtoc]

Issue

Fixes #24

Changes

• Add a zpty harness that drives a real interactive zsh -f session through Nova startup, async git redraws, and vi keymap changes.
• Verify that SIGKILLing the shell removes both its worker process and runtime directory.
• Run the harness in a dedicated Ubuntu CI job and document the local command.

Verification

• tests/zsh_e2e.zsh reports all four checks as successful.
• The git branch appears through a later ZLE redraw after the initial prompt.
• Killing the child shell leaves no worker process or runtime directory.

Additional Notes

• The harness uses zsh's built-in zpty module and adds no Rust dependency.
• It clears inherited XDG_RUNTIME_DIR so hosted runners keep worker files in the isolated fixture TMPDIR.
• The corrected harness passed 10 consecutive runs with a simulated hosted-runner XDG_RUNTIME_DIR.
• Local zsh-bench (16 runs): first prompt 29.1 ms, command lag 0.478 ms, input lag 0.516 ms.
• No additional manual zsh verification is required because the harness itself drives a real ZLE session.

[github-actions]

Nova zsh-bench

metric	median	previous	delta	status	green	yellow	orange	red
first prompt lag	33.02 ms	29.79 ms	+3.23 ms	yellow	25 ms	50 ms	100 ms	> 100 ms
first command lag	33.27 ms	29.95 ms	+3.32 ms	green	75 ms	150 ms	300 ms	> 300 ms
command lag	1.14 ms	0.58 ms	+0.56 ms	green	5 ms	10 ms	20 ms	> 20 ms
input lag	0.42 ms	0.58 ms	-0.16 ms	green	10 ms	20 ms	40 ms	> 40 ms

Previous is the stored benchmark for 9ce8bf3.
Delta is current median minus previous median. Lower is faster.

Red status fails CI. Values are medians from zsh-bench raw iterations.

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