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/skills.c

@@ -488,6 +488,16 @@ char *cst_skill_read(const char *ws, const char *name, const char *file,
   return cst_buf_steal(&b);
 }
 
+const char *cst_init_prompt(void) {
+  return "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.";
+}
+
 char *cst_skill_prompt(const char *ws, const char *name, const char *task) {
   int code = 0;
   char *body = cst_skill_read(ws, name, NULL, &code);