`supports_vision` is currently hardcoded per-provider. The same Ollama instance can run `llava` (vision) or
`codellama` (no vision), but the code fixes vision support at the provider level with no user override.
This adds a top-level `model_support_vision: Option<bool>` config key — tri-state:
- **Unset (default):** provider's built-in value, zero behavior change
- **`true`:** force vision on (e.g. Ollama + llava)
- **`false`:** force vision off
Follows the exact same pattern as `reasoning_enabled`. Override is applied at the wrapper layer (`ReliableProvider` /
`RouterProvider`) — no concrete provider code is touched.
## Changes
**Config surface:**
- Top-level `model_support_vision` field in `Config` struct with `#[serde(default)]`
- Env override: `ZEROCLAW_MODEL_SUPPORT_VISION` / `MODEL_SUPPORT_VISION`
**Provider wrappers (core logic):**
- `ReliableProvider`: `vision_override` field + `with_vision_override()` builder + `supports_vision()` override
- `RouterProvider`: same pattern
**Wiring (1-line each):**
- `ProviderRuntimeOptions` struct + factory functions
- 5 construction sites: `loop_.rs`, `channels/mod.rs`, `gateway/mod.rs`, `tools/mod.rs`, `onboard/wizard.rs`
**Docs (i18n parity):**
- `config-reference.md` — Core Keys table
- `providers-reference.md` — new "Ollama Vision Override" section
- Vietnamese sync: `docs/i18n/vi/` + `docs/vi/` (4 files)
## Non-goals
- Does not change any concrete provider implementation
- Does not auto-detect model vision capability
## Test plan
- [x] `cargo fmt --all -- --check`
- [x] `cargo clippy --all-targets -- -D warnings` (no new errors)
- [x] 5 new tests passing:
- `model_support_vision_deserializes` — TOML parse + default None
- `env_override_model_support_vision` — env var override + invalid value ignored
- `vision_override_forces_true` — ReliableProvider override
- `vision_override_forces_false` — ReliableProvider override
- `vision_override_none_defers_to_provider` — passthrough behavior
## Risk and Rollback
- **Risk:** Low. `None` default = zero behavior change for existing users.
- **Rollback:** Revert commit. Field is `#[serde(default)]` so old configs without it will deserialize fine.
(cherry picked from commit a1b8dee785)
3a38c80c05