docs: split user and developer docs (#93)

docs/errors.md

@@ -1,24 +0,0 @@
-# Errors and Result Serialization
-
-## Error taxonomy (`omnigraph::error::OmniError`)
-
-- `Compiler(...)` — schema/query parse/typecheck errors
-- `Lance(String)` — storage layer
-- `DataFusion(String)` — execution layer
-- `Io(io::Error)`
-- `Manifest(ManifestError { kind: BadRequest|NotFound|Conflict|Internal, details: Option<ManifestConflictDetails>, … })`
-  - `ManifestConflictDetails::ExpectedVersionMismatch { table_key, expected, actual }` — caller's `expected_table_versions` did not match the manifest's current latest non-tombstoned version (set by `OmniError::manifest_expected_version_mismatch`).
-  - `ManifestConflictDetails::RowLevelCasContention` — Lance row-level CAS rejected the publish because a concurrent writer landed the same `object_id`. Retried internally by the publisher; only surfaces if the retry budget exhausts.
-  - **D₂ parse-time rejection** (MR-794): a single mutation query that mixes inserts/updates with deletes errors out *before any I/O* with kind `BadRequest`. Message: `mutation '<name>' on the same query mixes inserts/updates and deletes; split into separate mutations: (1) inserts and updates, then (2) deletes`. See [docs/query-language.md](query-language.md) for the rule and [docs/runs.md](runs.md) for the underlying staged-write rationale.
-- `MergeConflicts(Vec<MergeConflict>)`
-
-Compiler-side `NanoError` covers parse / catalog / type / storage / plan / execution / arrow / lance / IO / manifest / unique-constraint, each with structured spans (`SourceSpan { start, end }`) for ariadne-style diagnostics.
-
-## Result serialization (`omnigraph_compiler::result::QueryResult`)
-
-- `to_arrow_ipc()` — efficient binary
-- `to_sdk_json()` — JS-safe JSON (large i64 wrapped in metadata)
-- `to_rust_json()` — Rust-friendly JSON
-- `batches()` — direct Arrow `RecordBatch` access
-
-Mutation results: `{ affectedNodes: usize, affectedEdges: usize }` (also exposed as a tiny Arrow batch).