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

CMakeLists.txt

@@ -51,6 +51,12 @@ set(CST_CORE_SOURCES
   src/os_compat.c
   src/sandbox.c
   src/sandbox_exec.c
+  src/keymap.c
+  src/skills.c
+  src/extensions.c
+  src/plugins.c
+  src/mcp.c
+  src/subagent.c
   src/session.c
   src/tools/tools.c
   src/tools/patch.c
@@ -167,7 +173,9 @@ endforeach()
 # Tests that link the whole core (no TUI).
 foreach(t test_llm_body test_sse_decode test_apply_patch test_session_roundtrip
           test_shell test_diff test_tools_fs test_registry
-          test_undo test_config test_models test_sandbox)
+          test_undo test_config test_models test_sandbox
+          test_keymap test_shell_compact test_skills
+          test_plugins test_mcp test_subagent)
   add_executable(${t} tests/${t}.c ${CST_CORE_SOURCES})
   target_include_directories(${t} PRIVATE include "${CJSON_DIR}")
   target_compile_definitions(${t} PRIVATE _GNU_SOURCE)