fix(opencode): respect HARNESS_MODEL in baked-in opencode.json (small_model path)

[AbirAbbas]

Summary

The baked-in `~/.config/opencode/opencode.json` hardcoded both `model` and `small_model` to a literal `openrouter/moonshotai/kimi-k2.5`. The main model is fine — the Python harness provider passes `-m $HARNESS_MODEL` to `opencode run`, which overrides config. But OpenCode's `small_model` (used for session titles, summarization, prompt compaction) reads from config, not the CLI flag. Result: a deployer setting `HARNESS_MODEL=X` on Railway gets X for the main model and `kimi-k2.5` for small_model — env var silently bypassed.

OpenCode supports `{env:VAR}` interpolation in string fields. This PR switches `model` and `small_model` to `{env:HARNESS_MODEL}` so both paths read from the same env var the rest of the pr-af stack already respects.

What changes

```diff

• "model":"openrouter/moonshotai/kimi-k2.5",
• "small_model":"openrouter/moonshotai/kimi-k2.5",
• "provider":{"openrouter":{"options":{"apiKey":"{env:OPENROUTER_API_KEY}"},"models":{"moonshotai/kimi-k2.5":{}}}}

• "model":"{env:HARNESS_MODEL}",
• "small_model":"{env:HARNESS_MODEL}",
• "provider":{"openrouter":{"options":{"apiKey":"{env:OPENROUTER_API_KEY}"}}}
  ```

The Dockerfile already sets `HARNESS_MODEL=openrouter/moonshotai/kimi-k2.5` as the image default, so a fresh container with no env overrides still has a value to interpolate. Railway / docker-compose env injection wins because it lands after the image's ENV layer.

The `models` registration block was dropped — OpenCode auto-handles unknown model names from a configured provider, and pinning a specific name there broke the moment `HARNESS_MODEL` changed.

Why this isn't the same as #23 (closed)

#23 was bumping the literal `kimi-k2.5` → `kimi-k2.6` in the same files. That was cosmetic — env vars already overrode the main-model path. This PR fixes the actual env-var bypass: the small_model field that env vars couldn't override at all.

Test plan

• `docker build .` succeeds
• On staging deploy with `HARNESS_MODEL` set, verify OpenRouter dashboard shows zero traffic to non-$HARNESS_MODEL models from this service's key

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