v0.8.4: WebUI sessions, diff, settings, slash commands and extensions

Sessions: rename, delete and search; /api/retry regenerates the last
answer, editing a prompt resends from there; /api/export downloads the
transcript as Markdown (cst_session_export_md). Session messages carry
note/tool_result markers, so a reload rebuilds the tool steps with
their arguments, output and exit status; subagent progress is shown.

Diff and settings: /api/diff shows the real diff; /api/settings covers
protocol, max_steps, tool switches and confirm; /api/permissions edits
the rules; sandbox status is reported. Session rename no longer reads
past a short title.

Commands: slash commands with completion in the composer (/model,
/provider, /new, /retry, /undo, /compact, /export, /diff, /init,
/skill and more), a Ctrl+P palette, Shift+Tab cycles the approval
mode, F1 or ? shows the keys. "Thinking expanded" is saved in the
settings (show_thinking), "expand all tool output" in the browser.

Extensions: GET /api/extensions lists MCP servers, plugins, skills,
agent roles and subagent runs; POST /api/extensions/reload and
/api/extensions/trust; a workspace switch reloads them. /api/init and
/api/skill run on the server (cst_init_prompt shared with the TUI).

Errors, statuses and [cst] notes are shown in Chinese with a readable
reason instead of rc=; a failed turn's error survives the reload.

Tests: scripts/e2e_webui_sessions.py, e2e_webui_workspace.py and
e2e_webui_ext.py, run from e2e_webui.sh.

src/tui.c

@@ -1042,10 +1042,7 @@ static int find_tool_block(tui_t *t, const char *id) {
   return -1;
 }
 
-static int msg_is_tool_result(const cst_message_t *m) {
-  return m && (m->role == CST_ROLE_TOOL ||
-               (m->role == CST_ROLE_USER && m->tool_call_id[0]));
-}
+#define msg_is_tool_result cst_message_is_tool_result
 
 /* Strip the "[tool …]" / "result <id> exit=N <n>" header and "end" trailer. */
 static void tool_result_view(const char *content, const char **body,
@@ -4774,12 +4771,6 @@ static void cmd_diff(tui_t *t, char *arg, int *quit) {
   free(df.out);
 }
 
-static void md_fence(FILE *fp, const char *s, size_t max) {
-  size_t n = strlen(s);
-  fprintf(fp, "````\n%.*s%s\n````\n\n", (int)(n > max ? max : n), s,
-          n > max ? "\n…(truncated)" : "");
-}
-
 static void cmd_export(tui_t *t, char *arg, int *quit) {
   (void)quit;
   const cst_agent_config_t *cfg = cst_agent_config(t->agent);
@@ -4799,44 +4790,20 @@ static void cmd_export(tui_t *t, char *arg, int *quit) {
     *sl = 0;
     os_mkdir_p(dir, 0755);
   }
-  FILE *fp = fopen(path, "wb");
+  char *md = cst_session_export_md(cst_agent_session(t->agent), t->title, sid,
+                                   cfg->model);
+  FILE *fp = md ? fopen(path, "wb") : NULL;
   if (!fp) {
+    free(md);
     push_system_line(t, "Error: cannot write %s", path);
     return;
   }
-  fprintf(fp, "# %s\n\nsession `%s` · model `%s`\n\n", t->title, sid,
-          cfg->model);
-  cst_session_t *s = cst_agent_session(t->agent);
-  cst_session_lock(s);
-  for (int i = 0; i < cst_session_count(s); i++) {
-    const cst_message_t *m = cst_session_get(s, i);
-    const char *c = m->content ? m->content : "";
-    if (m->role == CST_ROLE_SYSTEM)
-      continue;
-    if (msg_is_tool_result(m)) {
-      fprintf(fp, "**Tool result** `%s`\n\n", m->tool_name);
-      md_fence(fp, c, 4000);
-    } else if (cst_message_is_note(m)) {
-      fprintf(fp, "> %.*s\n\n", (int)strcspn(c, "\n"), c);
-    } else if (m->role == CST_ROLE_USER) {
-      fprintf(fp, "## User\n\n%s\n\n", c);
-      for (int k = 0; k < m->nattach; k++) {
-        char d[128];
-        cst_attach_describe(&m->attach[k], d, sizeof(d));
-        fprintf(fp, "- image `%s` (%s)\n", m->attach[k].name, d);
-      }
-      if (m->nattach)
-        fputc('\n', fp);
-    } else {
-      fprintf(fp, "## Assistant\n\n%s\n\n", c);
-      if (m->tool_calls_json && m->tool_calls_json[0]) {
-        fprintf(fp, "**Tool calls**\n\n");
-        md_fence(fp, m->tool_calls_json, 4000);
-      }
-    }
+  fputs(md, fp);
+  free(md);
+  if (fclose(fp) != 0) {
+    push_system_line(t, "Error: cannot write %s", path);
+    return;
   }
-  cst_session_unlock(s);
-  fclose(fp);
   push_system_line(t, "(exported to %s)", path);
 }
 
@@ -5013,14 +4980,7 @@ static void cmd_init(tui_t *t, char *arg, int *quit) {
   (void)arg;
   (void)quit;
   free(t->pending_prompt);
-  t->pending_prompt = xstrdup(
-      "Analyze this repository and write an AGENTS.md file at the workspace "
-      "root for future coding agents. Cover: what the project is, how to "
-      "build, test and run it (exact commands), the directory layout, coding "
-      "conventions you can infer from the code, and anything surprising. "
-      "Explore with list_dir/glob/grep/read_file before writing. Keep it "
-      "under 150 lines. If AGENTS.md already exists, improve it rather than "
-      "starting over.");
+  t->pending_prompt = xstrdup(cst_init_prompt());
 }
 
 static void cmd_compact(tui_t *t, char *arg, int *quit) {