Add an Effect composition root and make the agent runtime a service

[FreshlyBrewedCode]

Parent

#32

What to build

The daemon gets an Effect composition root, and the agent runtime is the first service to move into
it. server/daemon.ts builds a Layer; the agent runtime is resolved from context instead of being
threaded by hand through eight files and roughly twenty call sites (cli.ts → daemon.ts →
ServerOptions → StartTrackedRunOptions → DispatchEnv → startRun → buildAgentStepEffect).

Two concrete outcomes beyond deleting plumbing:

• The adapter becomes selectable from factory.config.ts. Today "pluggable adapter" is a claim
  the code does not support: there is one production implementation, chosen by a hardcoded
  opencodeAdapter default in two separate places, with no config surface at all.
• DispatchEnv loses a field. That bag exists to carry environment down a recursive dispatch
  chain; every field removed from it is one less thing a child run has to inherit explicitly.

The agent runtime goes first because its plumbing is the worst in the codebase — it proves the
pattern on the strongest case before #37 applies it to the remaining singletons.

Scope boundary, and it matters: server/http.ts's route handlers and sseStream stay plain
functions over Request/Response/ReadableStream. They read already-constructed services; they
do not become Effects. D22 is reaffirmed, not reopened.

ADRs to update as part of this ticket — both already carry the decision, so this is confirming the
text matches what shipped, and amending it if implementation forced a different shape:
docs/adr/0009-imperative-workflows-effect-boundary.md §5 and the 2026-09-18 status note in
docs/adr/0004-server-dispatch.md. The seam itself is docs/adr/0012-agent-runtime-seam.md §4.

Acceptance criteria

• The agent runtime is a Context.Service resolved from context, not a value passed through interfaces
• adapter is gone as a pass-through field from StartTrackedRunOptions, DispatchEnv, ServerOptions and the run/step options
• The agent runtime is selectable from factory.config.ts, with opencode as the default when unset
• server/http.ts's route handlers and SSE stream are still plain functions, not Effects
• Tests swap the corpus-replay runtime by providing a layer, not by threading an option
• Nested runs still inherit the agent runtime correctly — a grandchild run works
• ADR 0009 §5 and the ADR 0004 status note match what shipped
• bun run check passes

Blocked by

• Move chunk interpretation into the agent adapter #35 — the adapter interface has to be final before it becomes a service, or the service shape changes twice.