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_plugins.h

@@ -0,0 +1,51 @@
+#ifndef CST_PLUGINS_H
+#define CST_PLUGINS_H
+
+/*
+ * Plugins: <config>/plugins/<dir>/plugin.json (always loaded) and
+ * <ws>/.agent/plugins/<dir>/plugin.json (only for trusted workspaces).
+ *
+ *   {"name": "jira", "description": "...",
+ *    "tools": [{"name": "jira_search", "description": "...",
+ *               "parameters": {JSON Schema},
+ *               "command": ["python3", "search.py"],   (or a shell string)
+ *               "perm": "read|write|shell|net", "timeout": 60}],
+ *    "skills": "skills",
+ *    "mcpServers": {"name": {"command": "...", "args": [], "env": {}}}}
+ *
+ * A tool call runs the command in the workspace with the arguments as JSON
+ * on stdin (CST_PLUGIN_DIR / CST_WORKSPACE in the environment); stdout +
+ * stderr is the result and the exit status its exit code. Relative argv
+ * entries that exist in the plugin directory are made absolute.
+ */
+
+#include "os_compat.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#define CST_PLUGINS_MAX 32
+
+typedef struct {
+  char name[64];
+  char dir[OS_PATH_MAX];
+  char source[16]; /* "user" | "workspace" */
+  char desc[200];
+  int ntools;
+  int nmcp;
+  int nskills_root; /* 1 when it adds a skills directory */
+  char error[200];  /* load problem, "" when fine */
+} cst_plugin_info_t;
+
+/* Load plugins; workspace ones only when ws_trusted. Returns the number of
+ * plugins loaded; *untrusted (optional) counts skipped workspace plugins. */
+int cst_plugins_load(const char *workspace, int ws_trusted, int *untrusted);
+void cst_plugins_unload(void);
+int cst_plugins_info(cst_plugin_info_t *out, int max);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif