RuntimeConfig with embedding_model: None still seeds additional_embedding_models, hard-failing remember on model-less machines

[ohdearquant]

Bug

Setting embedding_model: None on RuntimeConfig does not produce an embedding-free runtime:

let config = RuntimeConfig {
    db_path: None,
    embedding_model: None, // intent: no embedders, text-only recall
    ..RuntimeConfig::default()
};

RuntimeConfig::default() (crates/khive-runtime/src/config.rs, Default impl) separately seeds additional_embedding_models with ParaphraseMultilingualMiniLmL12V2 whenever KHIVE_ADDITIONAL_EMBEDDING_MODELS is unset. The runtime therefore still registers an embedder, and memory.remember / memory.recall lazily initialize it on first use.

On a machine with local model files this is invisible. On a machine without them (CI runners, fresh installs), the first memory.remember hard-fails:

runtime: embedding: model initialization failed: IO error: No such file or directory

The failure is hard, not degraded: create_note_inner's single-model vector path propagates the embed error with full compensation (the note row and FTS doc are rolled back), so the write is lost.

Why it matters

• The obvious spelling of "no embeddings" (embedding_model: None) silently means "no default model, but one additional model". Every consumer that wants a deterministic no-embed runtime must know to also write additional_embedding_models: vec![].
• The trap is environment-dependent: tests written this way pass on developer machines and fail only on model-less runners. This surfaced in downstream integration tests that had been green for their entire life because the repository they lived in had no CI.
• The runtime's own test code already uses the two-field form (runtime.rs uses embedding_model: None together with an explicitly cleared additional list), which shows the intent exists but is not encoded in the API.

Repro

On a machine without model files under the khive data dir (or with HOME pointed at an empty directory):

let config = RuntimeConfig { db_path: None, embedding_model: None, ..RuntimeConfig::default() };
let rt = KhiveRuntime::new(config).unwrap();
// dispatch memory.remember → Err(Runtime(Embedding(ModelInitialization("IO error: ..."))))

Adding additional_embedding_models: vec![] makes the same code pass, with recall degrading to the FTS text leg.

Suggested fix (either)

1. Make embedding_model: None authoritative: when the primary model is explicitly None, do not seed additional_embedding_models from the env default (an explicit env var or field value still wins).
2. Or add a constructor/builder method (RuntimeConfig::no_embed() or similar) that produces the genuinely embedding-free config, and document the two-field requirement on both fields.

Option 1 matches the principle of least surprise; option 2 is non-breaking.