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/lock.rs

@@ -12,7 +12,7 @@ use tracing::{debug, error, info, warn};
 use uuid::Uuid;
 
 use super::db::create_connection;
-use super::error::{GiError, Result};
+use super::error::{LoreError, Result};
 use super::time::{ms_to_iso, now_ms};
 
 /// Maximum consecutive heartbeat failures before signaling error.
@@ -116,7 +116,7 @@ impl AppLock {
                 } else {
                     // Lock held by another active process - rollback and return error
                     drop(tx);
-                    return Err(GiError::DatabaseLocked {
+                    return Err(LoreError::DatabaseLocked {
                         owner: existing_owner,
                         started_at: ms_to_iso(acquired_at),
                     });