cli: add interactive split-footer mode to run

Add `run --interactive` (`-i`) which renders a split-screen
TUI inside the run command: scrollback output on top, input footer
on the bottom with prompt, permission, and question views.

This bridges the gap between headless `run` and the full TUI,
giving users an interactive experience without leaving the CLI
pipeline. Tool rendering, session data reduction, and stream
transport are extracted into run/ submodules to keep the
orchestration manageable.

*.shared.ts files contain shared logic for both run and the TUI, but
are intentionally kept in /run for now until later refactor

packages/opencode/src/cli/cmd/run/session.shared.ts

@@ -0,0 +1,94 @@
+// Session message extraction and prompt history.
+//
+// Fetches session messages from the SDK and extracts user turn text for
+// the prompt history ring. Also finds the most recently used variant for
+// the current model so the footer can pre-select it.
+import type { RunInput } from "./types"
+
+const LIMIT = 200
+
+export type SessionMessages = NonNullable<Awaited<ReturnType<RunInput["sdk"]["session"]["messages"]>>["data"]>
+
+type Turn = {
+  text: string
+  provider: string | undefined
+  model: string | undefined
+  variant: string | undefined
+}
+
+export type RunSession = {
+  first: boolean
+  turns: Turn[]
+}
+
+function text(msg: SessionMessages[number]): string {
+  return msg.parts
+    .filter((part) => part.type === "text")
+    .map((part) => part.text.trim())
+    .filter((part) => part.length > 0)
+    .join("\n")
+}
+
+function turn(msg: SessionMessages[number]): Turn | undefined {
+  if (msg.info.role !== "user") {
+    return
+  }
+
+  return {
+    text: text(msg),
+    provider: msg.info.model.providerID,
+    model: msg.info.model.modelID,
+    variant: msg.info.variant,
+  }
+}
+
+export function createSession(messages: SessionMessages): RunSession {
+  return {
+    first: messages.length === 0,
+    turns: messages.flatMap((msg) => {
+      const item = turn(msg)
+      return item ? [item] : []
+    }),
+  }
+}
+
+export async function resolveSession(sdk: RunInput["sdk"], sessionID: string, limit = LIMIT): Promise<RunSession> {
+  const response = await sdk.session.messages({
+    sessionID,
+    limit,
+  })
+  return createSession(response.data ?? [])
+}
+
+export function sessionHistory(session: RunSession, limit = LIMIT): string[] {
+  const out: string[] = []
+
+  for (const turn of session.turns) {
+    if (!turn.text) {
+      continue
+    }
+
+    if (out[out.length - 1] === turn.text) {
+      continue
+    }
+
+    out.push(turn.text)
+  }
+
+  return out.slice(-limit)
+}
+
+export function sessionVariant(session: RunSession, model: RunInput["model"]): string | undefined {
+  if (!model) {
+    return
+  }
+
+  for (let idx = session.turns.length - 1; idx >= 0; idx -= 1) {
+    const turn = session.turns[idx]
+    if (turn.provider !== model.providerID || turn.model !== model.modelID) {
+      continue
+    }
+
+    return turn.variant
+  }
+}