Phase 1 of the RAG driver abstraction (design doc sections 5.1-5.5a).
Data model:
- Add `driver: String` (serde default "yaml" via RagData::default_driver) and
`attached: bool` as the first two fields of RagData, so driver metadata sits
at the top of each RAG YAML. Old files without them load unchanged.
- Add `#[serde(default)]` to the non-Option fields so a minimal attached-RAG
YAML deserializes, and add `skip_serializing_if` to `vectors` so an empty
map renders no `vectors:` key.
- Add a hand-written `impl Default for RagData` delegating to `RagData::new()`.
It is deliberately not derived: a derived impl yields `driver: ""`, which is
not a valid driver string.
Validation (the price of the new serde defaults):
- Add `RagData::validate()`, called from `Rag::load()` after deserialization.
It enforces the (driver, attached) matrix and, critically, numeric floors
that the new defaults would otherwise mask: `top_k >= 1` unconditionally
(a 0 makes every query return nothing, silently), and `chunk_size >= 1` plus
`chunk_overlap < chunk_size` when not attached (a 0 chunk_size is a real
divide-by-zero panic while sizing embedding batches).
- Reject `.set rag_top_k 0` at the setter, before the set/update fork. Without
this, the new load-time floor turns one keystroke into an unloadable RAG:
the setter saves immediately and no dot-command can reach the file again.
Rebuild actually re-embeds now:
- `.rebuild rag` and `--rebuild-rag` previously re-scanned paths and re-embedded
nothing, because the content-hash skip fired regardless of the refresh flag.
Extract that decision into a module-level `find_hash_skip()` free function and
thread a `force_reingest` flag through `sync_documents()` and
`refresh_document_paths()`, set true only from `rebuild_rag()`. `.edit rag-docs`
stays incremental. Re-embedding costs time and API spend, so `rebuild_rag()`
now prints a one-line file-count warning first (no prompt: the path is
reachable from a non-interactive CLI flag).
Attached-RAG guards:
- Block `.rebuild rag` / `--rebuild-rag` and `.edit rag-docs` on attached RAGs,
which Coyote did not index and whose source documents it does not own.
- Add `Rag::driver()`, `Rag::is_attached()` and `Rag::file_count()`, and surface
driver/attached through `Rag::export()` so `.info rag` shows them.
Adds 12 unit tests (1299 -> 1311), including the two gate tests pinning that a
forced re-ingest does not hash-skip while an ordinary refresh still does.
Dark-Alex-17