simplify(llm): drop unconsumed adapter-change event and assembled call surfaces

The LLM service exposed three call surfaces (stream/streamBlocks/generate) but
the only production consumer — the agent loop — uses stream() exclusively,
feeding raw chunks through its own BlockAssembler for replay fidelity. Drop the
speculative convenience surfaces and the registry-change event that no listener
consumed, leaving stream() as the single model-call contract for both
production and tests.

- Remove LlmService.streamBlocks() and generate(), the llm/generate waterfall,
  and GenerateResult.
- Remove the llm/adapter-change event (declaration + emits) and the
  listener-throw rollback ordering that existed only to protect it; keep the
  HMR rollback disposer.
- Remove BlockAssembler.flushReady()/flushRemaining()/result() and the flushed
  cursor — the streaming-flush slice existed only for streamBlocks().
- Adapter tests drive a stream()+BlockAssembler helper (tests/assemble.ts)
  instead of generate(), exercising the same path production uses.
- Land the AGENTS.md "RFCs are proposals, not golden truth" principle and move
  both RFCs proposed -> implemented.

Implements:
- docs/rfc/implemented/simplification/2026-06-20-drop-unconsumed-llm-adapter-change-event.md
- docs/rfc/implemented/simplification/2026-06-20-drop-unconsumed-llm-assembled-surfaces.md

docs/architecture.md

@@ -45,7 +45,7 @@ Dependency rule: plugins depend on interface packages, never on `dsh-agent-loop`
 
 | ctx key | Class | Package | Role |
 |---|---|---|---|
-| `ctx.llm` | `LlmService` | dsh-llm | adapter registry; `stream()` / `streamBlocks()` / `generate()` |
+| `ctx.llm` | `LlmService` | dsh-llm | adapter registry; `stream()` |
 | `ctx.sessions` | `SessionStore` | dsh-session | creates/holds event-sourced `Session`s |
 | `ctx.sessionPersistence` | `SessionPersistence` (abstract) | dsh-session-persistence | durable persistence seam: create/append/load/list sessions |
 | `ctx.systemPrompt` | `SystemPrompt` | dsh-system-prompt | ordered sections + tool schemas → `assemble()` |