refactor: merge slashcommand tool into skill tool

Per reviewer feedback (code-yeongyu), keep the 'skill' tool as the main
tool and merge slashcommand functionality INTO it, rather than the reverse.

Changes:
- skill/tools.ts: Add command discovery (discoverCommandsSync) support;
  handle both SKILL.md skills and .omo/commands/ slash commands in a single
  tool; show combined listing in tool description
- skill/types.ts: Add 'commands' option to SkillLoadOptions
- skill/constants.ts: Update description to mention both skills and commands
- plugin/tool-registry.ts: Replace createSlashcommandTool with createSkillTool;
  register tool as 'skill' instead of 'slashcommand'
- tools/index.ts: Export createSkillTool instead of createSlashcommandTool
- plugin/tool-execute-before.ts: Update tool name checks from 'slashcommand'
  to 'skill'; update arg name from 'command' to 'name'
- agents/dynamic-agent-prompt-builder.ts: Categorize 'skill' tool as 'command'
- tools/skill-mcp/tools.ts: Update hint message to reference 'skill' tool
- hooks/auto-slash-command/executor.ts: Update error message

The slashcommand/ module files are kept (they provide shared utilities used
by the skill tool), but the slashcommand tool itself is no longer registered.

src/plugin/tool-registry.ts

@@ -12,7 +12,7 @@ import {
   createCallOmoAgent,
   createLookAt,
   createSkillMcpTool,
-  createSlashcommandTool,
+  createSkillTool,
   createGrepTools,
   createGlobTools,
   createAstGrepTools,
@@ -95,7 +95,7 @@ export function createToolRegistry(args: {
   })
 
   const commands = discoverCommandsSync(ctx.directory)
-  const slashcommandTool = createSlashcommandTool({
+  const skillTool = createSkillTool({
     commands,
     skills: skillContext.mergedSkills,
     mcpManager: managers.skillMcpManager,
@@ -129,7 +129,7 @@ export function createToolRegistry(args: {
     ...(lookAt ? { look_at: lookAt } : {}),
     task: delegateTask,
     skill_mcp: skillMcpTool,
-    slashcommand: slashcommandTool,
+    skill: skillTool,
     interactive_bash,
     ...taskToolsRecord,
     ...hashlineToolsRecord,