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/transcription/src/concurrency.rs

@@ -1,6 +1,6 @@
 use std::sync::Arc;
 
-use lumotia_core::error::{MagnotiaError, Result};
+use lumotia_core::error::{Error, Result};
 use lumotia_core::types::{AudioSamples, TranscriptionOptions};
 
 use crate::local_engine::{LocalEngine, TimedTranscript};
@@ -14,5 +14,5 @@ pub async fn run_inference(
 ) -> Result<TimedTranscript> {
     tokio::task::spawn_blocking(move || engine.transcribe_sync(&audio, &options))
         .await
-        .map_err(|e| MagnotiaError::TranscriptionFailed(format!("Task join error: {e}")))?
+        .map_err(|e| Error::TranscriptionFailed(format!("Task join error: {e}")))?
 }