v2: add optimistic UI while MCP prompt commands resolve #34860

Problem

V2 slash commands backed by MCP prompts can take noticeable time to resolve because command submission must call the MCP server's prompt resolution path before the prompt is admitted to the session.

Today there is no optimistic/pending UI for that resolution phase. If an MCP prompt command is slow, the submit experience can look like the prompt input is stalled or in an odd intermediate state before the resolved prompt appears in the session.

Expected behavior

After submitting an MCP prompt slash command, the TUI should immediately show an explicit pending/submitted command state while the server resolves the MCP prompt. The user should not be left looking at an apparently idle or stuck prompt input during that wait.

Reproduction aid

Temporarily add an artificial delay before MCP prompt evaluation:

diff --git a/packages/core/src/command.ts b/packages/core/src/command.ts
@@
-import { Context, Effect, Layer, Schema, Types } from "effect"
+import { Context, Duration, Effect, Layer, Schema, Types } from "effect"
@@
         const prompt = (yield* mcp.prompts()).find((prompt) => mcpCommandName(prompt.server, prompt.name) === input.name)
         if (!prompt) return yield* new NotFoundError({ command: input.name })
+        yield* Effect.sleep(Duration.seconds(3))
         const result = yield* mcp

Then submit an MCP prompt slash command with arguments. The UI should make the command resolution phase obvious and non-confusing during the artificial delay.