Surface OpenAI models in Opencode

[asujithan]

Fixes Issue #154

Root cause
OpenCode lists whatever providers ucode writes into opencode.json, and ucode never wrote a databricks-openai provider. Three layers were each missing a piece: want_codex didn't include "opencode", so codex models were never fetched for opencode launches. URL builder: build_opencode_base_urls had no "openai" entry. Provider config: agents/opencode.py:render_overlay had no third branch; helpers (_resolve_model_selector, default_model) didn't know about an openai bucket.

Fix

Mirrors Pi's already-working databricks-openai wiring — same gateway path, same provider topology:

Changes

• src/ucode/databricks.py — build_opencode_base_urls now returns "openai": …/ai-gateway/codex/v1
• src/ucode/cli.py — want_codex includes "opencode" in tools; populates opencode_models["openai"] from codex_models
• src/ucode/agents/init.py — _TOOL_DISCOVERY_SOURCES["opencode"] includes "codex" so availability errors surface the codex discovery reason
• src/ucode/agents/opencode.py — third provider branch (databricks-openai via @ai-sdk/openai); PROVIDER_KEYS, _resolve_model_selector, and default_model extended; preference order now Claude → OpenAI → Gemini (matches Pi)

Test plan

• uv run ruff check . clean
• uv run pytest — 769 passed, 30 skipped; 2 pre-existing env failures unrelated (sandbox FS + Node v24/undici incompatibility, identical on main)
• 10 new focused unit tests
• tests/conftest.py e2e fixture extended so any e2e test using e2e_state["opencode_models"] now exercises the openai bucket