AppError::code() Duplicates ErrorCode Serde Serialization

Problem Statement

ErrorCode uses #[serde(rename_all = "SCREAMING_SNAKE_CASE")] for automatic serialization. But AppError::code() is a manually maintained match that maps enum variants to the same hardcoded strings. When a new error code is added, it must be added in 3 places: the enum, the serde attribute (automatic), and the code() match. The main.rs response builder uses e.code() (the string method) rather than serializing through serde, bypassing the single source of truth.

Findings

File: crates/core/src/error.rs:107-126

pub fn code(&self) -> &str {
    match self {
        AppError::PermissionDenied => "PERM_DENIED",
        AppError::ElementNotFound  => "ELEMENT_NOT_FOUND",
        // ... manually duplicated
    }
}

Proposed Solutions

Option A: Derive code() from serde (Recommended)

pub fn code(&self) -> String {
    serde_json::to_value(self.error_code())
        .and_then(|v| v.as_str().map(String::from)
        .unwrap_or("INTERNAL")
}

Effort: Small
Risk: Low — eliminates manual maintenance

Option B: Use strum's Display derive

Add strum crate, derive Display with SCREAMING_SNAKE_CASE transform on ErrorCode.

Effort: Tiny (add dependency)
Risk: Low

Recommended Action

Option A: derive from serde. No new dependencies.

Technical Details

File: crates/core/src/error.rs:107-126

Acceptance Criteria

AppError::code() is not a manual match
Adding a new error code variant requires exactly one change location

Work Log

2026-02-19: Finding identified by architecture-strategist review agent

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