Delete the unprepared prompt request

[SaladDay]

Core starts every Run with execution_prepare and then execution_start. Both callers of Dispatcher.deliver already passed a prepared handle, so the unprepared prompt_request path was unreachable. This deletes it end to end.

Core–Runtime protocol

• Deletes the prompt_request message and its rows in docs/runtime-protocol.md and the zh mirror.
• PromptRequestPayload is now only the execution_prepare configuration. The ConversationID field is deleted, since nothing read it.
• ReleaseOnCompletion is deleted in favor of WorkspaceReadOnly, which always carried the same value.
• validateExecutionEnvironment now owns the exactly-one local_environment / disable_execution_environment rule. Drop unsent Harness options and reject prompts without an environment #468 placed it beside that function only because prompt_request fixtures sent neither.
• The done row now describes the prepared ordering: native settlement, then the in-flight receipt send.

Daemon: deletes dispatch/prompt.go, output.go, the retained idle-session pool, releaseCompletedSession, finishSteering, emitTerminalError, and the unprepared branches in cancel, shutdown, suspend, workspace write and runtime preparation. Executor idle expiry is unchanged.

Core: deliver requires the prepared handle.

Tests

• Dispatch tests that asserted live behavior now run through prepare/start: steering, cancellation, receipts, function tools, MCP HTTP, interactions, suspend, shutdown and workspace.
• Deleted: tests that covered only prompt_request, and one timing-racy idle test whose guard is covered deterministically elsewhere.
• One dropped clause: "native release waits for the receipt". The prepared design settles natively first, and TestPreparedHandoffReleaseWaitsForMutationReceipt asserts that ordering.

Checks

• gofmt, build, vet (./apps/daemon/... ./internal/... ./services/core/...)
• dispatch with -race -count=2
• focused tests for transport, agentdaemon, execution, runtimegateway, localworkspace, the agents, cli and wireconformance
• the 14 touched integration tests against a test DB
• make check-runtime-contract, check-names, translations

Blind review: no blockers. Its fixes are in this commit. Two follow-ups get their own PRs: deleting the direct-call Harness factory (Runtime–Harness protocol) and a Shutdown flake that already occurs on main.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}