feat(opencode-agents): render agent instruction templates

[apucacao]

Decision

Render {{variables}} in non-system config messages with parse_template. History and the user turn stay raw. Instruction rendering, including the judge prompt, is already on #179, which is merged to main.

Why

Configured messages were posted with placeholders still in them. The other handlers render those messages. History and the latest user turn are already concrete text.

What this PR does not do

• It does not treat the model resolver as a writer of config["instructions"]. ModelResolver only returns a provider id and a model id.
• It does not add a messages-mode route.
• It does not change the session lifecycle from feat(opencode-agents): open one OpenCode session per call #179.
• It does not choose the AI Config key.

Next

Observability #2547 passes base_url to create_opencode_handler. This PR's base is main.

  

[jeffdupont]

LGTM — rendering once in the handler matches the runner contract ("the handler owns the single parse_template pass") and the claude/langchain/openai handlers. A couple of things worth addressing, non-blocking:

1. Resolver-override claim in the docstring (handler.py:694-695): it says rendering happens after the model resolver "so a caller system-prompt override written onto config is what gets rendered", but ModelResolver is Callable[[dict], tuple[str, str]] and nothing writes config["instructions"]. That only works if a custom resolver mutates its input — an undocumented side effect with no test. If downstream relies on it, please make it an explicit contract (documented on ModelResolver + a test where the resolver writes config["instructions"] = "Override {{x}}"), or preferably a dedicated override param; otherwise drop the claim.

2. Configured messages aren't rendered (_user_prompt, handler.py:704-735): non-system config["messages"] go through compose_history without parse_template, unlike claude-agents (handler.py:369,388) and openai-agents (handler.py:178). Since collapse_messages_to_instructions leaves messages intact when instructions is set, "Assist {{caller_email}}" would reach OpenCode literally. Render config messages (keeping history/user input raw), or document that agent mode doesn't template them.

3. Tests: consider covering variables=None, nested {{user.name}}, an unknown placeholder left intact, and that set_input_content_attributes receives the rendered text.

Note this should land before (or with) #179, which removes the skip_judges guidance.

[cursor]

Addressed in fabccbd. Left unmerged because the base is still #179.

1. Dropped the claim that the model resolver writes config["instructions"]. Nothing in this package does that, and it is not part of ModelResolver.
2. Non-system config messages go through parse_template. History and the user turn stay raw.
3. Tests cover variables=None, nested {{user.name}}, an unknown placeholder left intact, and gen_ai.system_instructions seeing the rendered text.

OpenCode tests: 44 passed.