refactor(core): Rename GiError to LoreError and add search infrastructure

Mechanical rename of GiError -> LoreError across the core module to
match the project's rebranding from gitlab-inbox to gitlore/lore.
Updates the error enum name, all From impls, and the Result type alias.

Additionally introduces:

- New error variants for embedding pipeline: OllamaUnavailable,
  OllamaModelNotFound, EmbeddingFailed, EmbeddingsNotBuilt. Each
  includes actionable suggestions (e.g., "ollama serve", "ollama pull
  nomic-embed-text") to guide users through recovery.

- New error codes 14-16 for programmatic handling of Ollama failures.

- Savepoint-based migration execution in db.rs: each migration now
  runs inside a SQLite SAVEPOINT so a failed migration rolls back
  cleanly without corrupting the schema_version tracking. Previously
  a partial migration could leave the database in an inconsistent
  state.

- core::backoff module: exponential backoff with jitter utility for
  retry loops in the embedding pipeline and discussion queues.

- core::project module: helper for resolving project IDs and paths
  from the local database, used by the document regenerator and
  search filters.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/core/error.rs

@@ -21,6 +21,9 @@ pub enum ErrorCode {
     TransformError,
     IoError,
     InternalError,
+    OllamaUnavailable,
+    OllamaModelNotFound,
+    EmbeddingFailed,
 }
 
 impl std::fmt::Display for ErrorCode {
@@ -39,6 +42,9 @@ impl std::fmt::Display for ErrorCode {
             Self::TransformError => "TRANSFORM_ERROR",
             Self::IoError => "IO_ERROR",
             Self::InternalError => "INTERNAL_ERROR",
+            Self::OllamaUnavailable => "OLLAMA_UNAVAILABLE",
+            Self::OllamaModelNotFound => "OLLAMA_MODEL_NOT_FOUND",
+            Self::EmbeddingFailed => "EMBEDDING_FAILED",
         };
         write!(f, "{code}")
     }
@@ -61,13 +67,16 @@ impl ErrorCode {
             Self::MigrationFailed => 11,
             Self::IoError => 12,
             Self::TransformError => 13,
+            Self::OllamaUnavailable => 14,
+            Self::OllamaModelNotFound => 15,
+            Self::EmbeddingFailed => 16,
         }
     }
 }
 
 /// Main error type for gitlore.
 #[derive(Error, Debug)]
-pub enum GiError {
+pub enum LoreError {
     #[error("Config file not found at {path}. Run \"lore init\" first.")]
     ConfigNotFound { path: String },
 
@@ -129,9 +138,25 @@ pub enum GiError {
 
     #[error("{0}")]
     Other(String),
+
+    #[error("Cannot connect to Ollama at {base_url}. Is it running?")]
+    OllamaUnavailable {
+        base_url: String,
+        #[source]
+        source: Option<reqwest::Error>,
+    },
+
+    #[error("Ollama model '{model}' not found. Run: ollama pull {model}")]
+    OllamaModelNotFound { model: String },
+
+    #[error("Embedding failed for document {document_id}: {reason}")]
+    EmbeddingFailed { document_id: i64, reason: String },
+
+    #[error("No embeddings found. Run: lore embed")]
+    EmbeddingsNotBuilt,
 }
 
-impl GiError {
+impl LoreError {
     /// Get the error code for programmatic handling.
     pub fn code(&self) -> ErrorCode {
         match self {
@@ -152,6 +177,10 @@ impl GiError {
             Self::NotFound(_) => ErrorCode::GitLabNotFound,
             Self::Ambiguous(_) => ErrorCode::GitLabNotFound,
             Self::Other(_) => ErrorCode::InternalError,
+            Self::OllamaUnavailable { .. } => ErrorCode::OllamaUnavailable,
+            Self::OllamaModelNotFound { .. } => ErrorCode::OllamaModelNotFound,
+            Self::EmbeddingFailed { .. } => ErrorCode::EmbeddingFailed,
+            Self::EmbeddingsNotBuilt => ErrorCode::EmbeddingFailed,
         }
     }
 
@@ -193,7 +222,15 @@ impl GiError {
             Self::Ambiguous(_) => Some(
                 "Use -p to choose a specific project.\n\n  Example:\n    lore issues 42 -p group/project-a\n    lore mrs 99 -p group/project-b",
             ),
-            _ => None,
+            Self::OllamaUnavailable { .. } => Some("Start Ollama: ollama serve"),
+            Self::OllamaModelNotFound { .. } => {
+                Some("Pull the model: ollama pull nomic-embed-text")
+            }
+            Self::EmbeddingFailed { .. } => {
+                Some("Check Ollama logs or retry with 'lore embed --retry-failed'")
+            }
+            Self::EmbeddingsNotBuilt => Some("Generate embeddings first: lore embed"),
+            Self::Json(_) | Self::Io(_) | Self::Transform(_) | Self::Other(_) => None,
         }
     }
 
@@ -227,12 +264,12 @@ pub struct RobotErrorOutput {
     pub error: RobotError,
 }
 
-impl From<&GiError> for RobotErrorOutput {
-    fn from(e: &GiError) -> Self {
+impl From<&LoreError> for RobotErrorOutput {
+    fn from(e: &LoreError) -> Self {
         Self {
             error: e.to_robot_error(),
         }
     }
 }
 
-pub type Result<T> = std::result::Result<T, GiError>;
+pub type Result<T> = std::result::Result<T, LoreError>;