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

Makefile

@@ -57,6 +57,9 @@ CORE_SRC = \
 	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 \
@@ -99,7 +102,13 @@ TESTS = \
 	tests/test_undo \
 	tests/test_config \
 	tests/test_models \
-	tests/test_sandbox
+	tests/test_sandbox \
+	tests/test_keymap \
+	tests/test_shell_compact \
+	tests/test_skills \
+	tests/test_plugins \
+	tests/test_mcp \
+	tests/test_subagent
 
 TEST_LIBS = -lm $(CURL_LIBS) $(PTHREAD_LIBS)