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

vendor/cjson/cJSON.c

@@ -89,7 +89,14 @@ typedef struct {
     const unsigned char *json;
     size_t position;
 } error;
+/* cst-agent: per thread, parses run concurrently (sub-agents, MCP) */
+#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
+static _Thread_local error global_error = { NULL, 0 };
+#elif defined(__GNUC__)
+static __thread error global_error = { NULL, 0 };
+#else
 static error global_error = { NULL, 0 };
+#endif
 
 CJSON_PUBLIC(const char *) cJSON_GetErrorPtr(void)
 {