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

include/cst_ext.h

@@ -0,0 +1,31 @@
+#ifndef CST_EXT_H
+#define CST_EXT_H
+
+/* Startup / reload of everything that adds tools at runtime: the skill
+ * tool, plugin exec tools and MCP servers. */
+
+#include "cst_agent.h"
+
+#include <stddef.h>
+#include <stdio.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/* Load for cfg's workspace (no-op for `--tools minimal`). One-line summary
+ * in report, e.g. "2 skills · 1 plugin (3 tools) · mcp 1/2". */
+void cst_extensions_load(const cst_agent_config_t *cfg, char *report,
+                         size_t report_sz);
+/* Summary of the last load (empty when nothing was found). */
+const char *cst_extensions_report(void);
+/* Headless: wait for MCP handshakes, then note servers that failed. */
+void cst_extensions_wait(FILE *log);
+/* Drop plugin/MCP tools and stop MCP servers. */
+void cst_extensions_unload(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif