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/main.c

@@ -1,4 +1,5 @@
 #include "cst_agent.h"
+#include "cst_ext.h"
 #include "cst_tools.h"
 #include "cst_version.h"
 #include "os_compat.h"
@@ -540,6 +541,14 @@ int main(int argc, char **argv) {
           cfg.api_base, cfg.model, cfg.effort, cst_protocol_name(cfg.protocol),
           cst_tool_profile_name(cfg.tool_profile), cfg.max_steps);
 
+  {
+    char ext[256];
+    cst_extensions_load(&cfg, ext, sizeof(ext));
+    if (ext[0] && cfg.headless)
+      fprintf(stderr, "extensions: %s\n", ext);
+    if (cfg.headless)
+      cst_extensions_wait(stderr);
+  }
   cst_agent_t *agent = cst_agent_create(&cfg);
   if (!agent) {
     fprintf(stderr, "failed to create agent (session path?)\n");
@@ -571,6 +580,7 @@ int main(int argc, char **argv) {
   }
 
   cst_agent_destroy(agent);
+  cst_extensions_unload();
   curl_global_cleanup();
   return rc;
 }