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/mod.rs

@@ -1,12 +1,14 @@
 //! Core infrastructure modules.
 
+pub mod backoff;
 pub mod config;
 pub mod db;
 pub mod error;
 pub mod lock;
 pub mod paths;
 pub mod payloads;
+pub mod project;
 pub mod time;
 
 pub use config::Config;
-pub use error::{GiError, Result};
+pub use error::{LoreError, Result};