agent: lumotia-rebrand — drop MagnotiaError prefix, now lumotia_core::Error

Phase 3 of the rebrand cascade per locked decision D4. MagnotiaError ->
Error in crates/core/src/error.rs (the crate name already qualifies it).
92 usages across 14 .rs files renamed via word-boundary sed.

One collision required disambiguation: lumotia_storage already had its
own local Error type (introduced by the slop-pass Area A residuals work).
crates/storage/src/error.rs aliases the imported core error as CoreError
on import; the From<Error> for CoreError boundary impl and the
CoreError::Storage construction site use the alias.

cargo build --workspace passes. cargo test --workspace: 330 pass, 0 fail.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/audio/src/concurrency.rs

@@ -16,6 +16,6 @@ pub async fn decode_and_resample(path: &Path) -> Result<AudioSamples> {
     })
     .await
     .map_err(|e| {
-        lumotia_core::error::MagnotiaError::AudioDecodeFailed(format!("Task join error: {e}"))
+        lumotia_core::error::Error::AudioDecodeFailed(format!("Task join error: {e}"))
     })?
 }