Extensions: skills, plugins, MCP, sub-agents; configurable keys

- Skills (SKILL.md discovery, skill tool, /skills /skill)
- plugin.json exec tools with permission prompts, timeouts and workspace trust (/plugins)
- MCP stdio client: JSON-RPC initialize/tools/list/tools/call, mcp__server__tool, /mcp and sidebar status
- task tool with general/explore/plan and .agent/agents/*.md roles, parallel read-only runs, cancel propagation, /agents
- Keymap with Ctrl+X leader, keybinds.json overrides and /keys panel
- Collapsed thinking, throttled streaming Markdown, leaner tool output, parallel read-only tools
- Spawn: close-on-exec pipes and a process-wide fork lock
- Tests for keymap, skills, plugins, MCP, sub-agents, shell compaction; docs/plugins.md

src/llm_openai.c

@@ -9,6 +9,7 @@
 #include "os_compat.h"
 
 #include <curl/curl.h>
+#include <stdatomic.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -477,9 +478,9 @@ static void sse_reset(sse_t *s) {
 
 /* --- record / replay ----------------------------------------------------- */
 
-static int g_trace_seq;
+static atomic_int g_trace_seq;
 
-void cst_llm_trace_reset(void) { g_trace_seq = 0; }
+void cst_llm_trace_reset(void) { atomic_store(&g_trace_seq, 0); }
 
 static void trace_path(const char *dir, int seq, const char *ext, char *out,
                        size_t n) {
@@ -575,7 +576,7 @@ int cst_llm_chat_stream(const cst_llm_request_t *req, char *err,
     return -1;
   }
 
-  int seq = g_trace_seq++;
+  int seq = atomic_fetch_add(&g_trace_seq, 1);
   const char *rec_dir = getenv("CST_RECORD_SSE");
   const char *rep_dir = getenv("CST_REPLAY_SSE");
   if (rec_dir && rec_dir[0]) {