Add tests for internal/zed init and doctor

[Jason-Abbott]

Description

internal/zed has only 17.0% test coverage. Init and doctor logic need tests.

TODO

• Add tests for zed init (extension.toml generation, Cargo.toml scaffolding)
• Add tests for zed doctor checks (extension exists, required files present)
• Add tests for workflow injection
• Target >40% coverage

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