## Summary
- classify plugin store and remote bundle I/O failures by privacy-safe
operation context
- include the normalized `sub_error_type` in structured plugin-install
failure warnings
- leave analytics events and schemas unchanged
## Why
Top-level error types such as `store_io` collapse multiple filesystem
operations, making logs harder to diagnose. The subtype identifies the
failed operation without logging paths or new exception details.
## Impact
Plugin-install warning logs gain an optional normalized subtype.
Analytics payloads, app-server APIs, and stored data are unchanged.
## Validation
- `just fmt`
- `just test -p codex-core-plugins -p codex-app-server` (1,235 passed;
14 failed and 6 timed out because local `sandbox-exec` was denied,
`test_stdio_server` was unavailable, or user-level skill fixtures were
loaded)
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