Move chunk interpretation into the agent adapter

[FreshlyBrewedCode]

Parent

#32

What to build

The AgentAdapter interprets its own chunk stream, so opencode's event vocabulary stops leaking
into the runtime. Today runtime/agent-step.ts string-matches CUSTOM + opencode.session-id to
populate AgentStepFinished.sessionId, and string-matches structured-output.complete — which is
not a provider capability at all, but @tanstack/ai-opencode's text adapter simulating
structured output by injecting JSON Schema into the prompt and re-parsing its own final message
(ADR 0001 §1).

After this ticket, the adapter yields the opaque chunk and a normalized signal drawn from a
small closed union Factory owns — session identifier, structured-output value, run error. The
runtime records those signals and never again matches a vendor event name. The union stays tiny by
rule: a member exists only when Factory has a field for it (AgentStepFinished.sessionId,
.output, .error), not as a general capability model for agents.

Chunks themselves do not change shape. ADR 0003 §2 stands: AgentChunk carries AG-UI verbatim, the
SPA keeps folding it with StreamProcessor from @tanstack/ai/client, and no stored event needs
migrating.

The observable payoff is that a second adapter — claude-code, codex, a raw provider loop — becomes
a normal amount of work instead of requiring it to emit opencode's exact CUSTOM event names. The
corpus replay adapter is the proof: it should be able to supply signals without replaying opencode
chunk shapes.

ADR 0012 was written ahead of this ticket — see docs/adr/0012-agent-runtime-seam.md §1, §2
and §5 for the boundary this implements, including why the adapter contract stays plain async
(AsyncIterable, not Stream.Stream).

Acceptance criteria

• AgentAdapter yields opaque chunks plus a normalized signal union; the union's members are documented against the event fields they populate
• No vendor event-name string (opencode.session-id, structured-output.complete) appears anywhere under src/runtime/ outside opencode-adapter.ts
• AgentStepFinished.sessionId, .output and .error are populated exactly as before for a live opencode run
• The runtime's two-tier structured-output resolution (event value, then finalText re-parse) still works — this ticket changes where the value comes from, not the fallback
• Existing corpus traces still replay without modification, and the replay adapter supplies signals without imitating opencode chunk shapes
• The adapter contract remains plain async (AsyncIterable), authorable without Effect
• bun run check passes

Blocked by

None (can start immediately).