fix(shell): default zsh dispatch to forgecode

[KooshaPari]

Summary

• default Zsh plugin setup and doctor diagnostics to forgecode
• preserve explicit FORGE_BIN overrides
• add focused shell-dispatch regression coverage

Validation

• zsh scripts/test-shell-dispatch.sh (8/8)

Provenance: f17756c7aa59ee5246d540f531ed2f4b1f08756f

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