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/session.c

@@ -8,6 +8,7 @@
 #include "cJSON.h"
 #include "cst_agent.h"
 #include "cst_attach.h"
+#include "cst_util.h"
 
 #include <pthread.h>
 #include <stdio.h>
@@ -409,3 +410,62 @@ int cst_session_rewrite(cst_session_t *s, const cst_message_t *msgs, int n) {
   free(copy);
   return rc;
 }
+
+int cst_message_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]));
+}
+
+static void md_fence(cst_buf_t *b, const char *s, size_t max) {
+  size_t n = strlen(s);
+  if (n > max)
+    n = cst_utf8_floor(s, max);
+  cst_buf_puts(b, "````\n");
+  cst_buf_append(b, s, n);
+  cst_buf_puts(b, n < strlen(s) ? "\n…(truncated)\n````\n\n" : "\n````\n\n");
+}
+
+char *cst_session_export_md(cst_session_t *s, const char *title,
+                            const char *sid, const char *model) {
+  if (!s)
+    return NULL;
+  cst_buf_t b = {0};
+  cst_buf_printf(&b, "# %s\n\nsession `%s` · model `%s`\n\n",
+                 title ? title : "", sid ? sid : "", model ? model : "");
+  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 (cst_message_is_tool_result(m)) {
+      cst_buf_printf(&b, "**Tool result** `%s`\n\n", m->tool_name);
+      md_fence(&b, c, 4000);
+    } else if (cst_message_is_note(m)) {
+      cst_buf_puts(&b, "> ");
+      cst_buf_append(&b, c, strcspn(c, "\n"));
+      cst_buf_puts(&b, "\n\n");
+    } else if (m->role == CST_ROLE_USER) {
+      cst_buf_puts(&b, "## User\n\n");
+      cst_buf_puts(&b, c);
+      cst_buf_puts(&b, "\n\n");
+      for (int k = 0; k < m->nattach; k++) {
+        char d[128];
+        cst_attach_describe(&m->attach[k], d, sizeof(d));
+        cst_buf_printf(&b, "- image `%s` (%s)\n", m->attach[k].name, d);
+      }
+      if (m->nattach)
+        cst_buf_puts(&b, "\n");
+    } else {
+      cst_buf_puts(&b, "## Assistant\n\n");
+      cst_buf_puts(&b, c);
+      cst_buf_puts(&b, "\n\n");
+      if (m->tool_calls_json && m->tool_calls_json[0]) {
+        cst_buf_puts(&b, "**Tool calls**\n\n");
+        md_fence(&b, m->tool_calls_json, 4000);
+      }
+    }
+  }
+  cst_session_unlock(s);
+  return cst_buf_steal(&b);
+}