refactor(executor): delete the dead GeminiRunner stub and its sandbox helpers (Phase 4)

The production "gemini" agent type is served by ContainerRunner (Docker), not by
the standalone subprocess GeminiRunner, which was never wired into serve.go and
returned canned/simulated output — the dangerous landmine called out in
CLAUDE.md's design debt. Removing it (gemini.go + tests).

The subprocess sandbox helpers (setupSandbox/teardownSandbox/sandboxCloneSource
in sandbox.go), preserved in Phase 2 only because GeminiRunner used them, are now
orphaned — ContainerRunner has its own git/workspace handling — so they go too.

No production behavior change: nothing instantiated GeminiRunner. Build green;
executor tests unchanged except the pre-existing git commit-signing failures.

https://claude.ai/code/session_01SESwn7kQ7oP62trWw6pc39

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