feat(hub): session transcript 连续性 + slash commands (/new /resume /reset)

修复每次 @bot 都是失忆状态的问题。
- transcript.ts: JSONL 文件存 session 对话消息(workspace/.cph/sessions/
  <id>.jsonl),append-only;和 Claude Code 一致;ADR-0003 说的'不存全
  历史'是群聊历史,session 对话是 Hub 自己产生的,不冲突
- runner.ts: RunRequest 加 transcriptPath;run 开始时读历史拼进初始
  messages(system prompt 只在首条);run 结束时 append 本轮新消息
  (loadedCount 区分历史 vs 新增,不重复写);best-effort(写失败不丢
  run 结果)
- trigger.ts: slash commands 在 lock 检查之前(/new /resume /reset
  不创建 run 不需要锁);transcriptPath 传给 runner
  /new = 归档当前 session,下次 @bot 自动建新的
  /resume = 恢复最近归档的 session
  /reset = 同 /new
  未知 /cmd 透传给 agent 当普通 prompt
- unit: transcript.test.ts 读写回环/append 累积/损坏行恢复/路径(6)
- integration: trigger.test.ts 加 /new /resume /reset /unknown(4)
修了 slash command 被 lock 挡住的问题(提到 lock 之前)。
vitest run 45/45 通过;tsc rc=0。

hub/src/agent/transcript.ts

@@ -0,0 +1,64 @@
+/**
+ * Session transcript — append-only JSONL file in the workspace.
+ *
+ * Each line is one {@link Message}, JSON-encoded. The transcript lives at
+ * `workspaceDir/.cph/sessions/<sessionId>.jsonl`, following the same "workspace
+ * is a directory tree" principle as the engineering file itself (ADR-0007).
+ *
+ * This mirrors how Claude Code stores its transcript (a `.jsonl` file per
+ * session). The agent can read its own history via `read_file` — no special
+ * "load history" code path. ADR-0002's lock guarantees one run per project at a
+ * time, so the transcript is never concurrently written.
+ *
+ * ADR-0003: this stores **agent session messages** (prompt + response + tool
+ * calls produced by the Hub), NOT Feishu group chat history. The two are
+ * distinct; storing session messages does not conflict with "the Hub avoids
+ * becoming a full chat history store."
+ */
+import { readFile, writeFile, mkdir, appendFile } from "node:fs/promises";
+import { join, dirname } from "node:path";
+import type { Message } from "./provider.js";
+
+/// Subdirectory within a workspace where session transcripts live.
+export const TRANSCRIPT_DIR = ".cph/sessions";
+
+/** Path for a session's transcript file within a workspace. */
+export function transcriptPath(workspaceDir: string, sessionId: string): string {
+  return join(workspaceDir, TRANSCRIPT_DIR, `${sessionId}.jsonl`);
+}
+
+/**
+ * Read prior messages from a transcript file. Returns an empty array if the
+ * file doesn't exist yet (first run in this session). Corrupt lines are
+ * skipped — a partially-written last line (crash mid-append) won't break the
+ * next run.
+ */
+export async function readTranscript(path: string): Promise<Message[]> {
+  let content: string;
+  try {
+    content = await readFile(path, "utf8");
+  } catch {
+    return [];
+  }
+  const messages: Message[] = [];
+  for (const line of content.split("\n")) {
+    const trimmed = line.trim();
+    if (trimmed === "") continue;
+    try {
+      messages.push(JSON.parse(trimmed) as Message);
+    } catch {
+      // Skip corrupt line — likely a partial write from a crash.
+    }
+  }
+  return messages;
+}
+
+/**
+ * Append messages to a transcript file. Creates the parent directory if needed.
+ * Each message is written as one JSON line.
+ */
+export async function appendTranscript(path: string, messages: readonly Message[]): Promise<void> {
+  await mkdir(dirname(path), { recursive: true });
+  const lines = messages.map((m) => JSON.stringify(m)).join("\n") + "\n";
+  await appendFile(path, lines, "utf8");
+}