refactor(loaders): migrate to async-first pattern for commands and skills

- Remove all sync functions from command loader (async now default)
- Remove sync load functions from skill loader (async now default)
- Add resolveSymlinkAsync to file-utils.ts
- Update all callers to use async versions:
  - config-handler.ts
  - index.ts
  - tools/slashcommand/tools.ts
  - tools/skill/tools.ts
  - hooks/auto-slash-command/executor.ts
  - loader.test.ts
- All 607 tests pass, build succeeds

Generated with assistance of 🤖 [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/features/opencode-skill-loader/loader.test.ts

@@ -53,7 +53,7 @@ This is the skill body.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "test-skill")
 
         // #then
@@ -89,7 +89,7 @@ This is a simple skill.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "simple-skill")
 
         // #then
@@ -122,7 +122,7 @@ Skill with env vars.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "env-skill")
 
         // #then
@@ -149,7 +149,7 @@ Skill body.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         // #then - when YAML fails, skill uses directory name as fallback
         const skill = skills.find(s => s.name === "bad-yaml-skill")
 
@@ -186,7 +186,7 @@ Skill body.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "ampcode-skill")
 
         // #then
@@ -227,7 +227,7 @@ Skill body.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "priority-skill")
 
         // #then - mcp.json should take priority
@@ -259,7 +259,7 @@ Skill body.
       process.chdir(TEST_DIR)
 
       try {
-        const skills = discoverSkills({ includeClaudeCodePaths: false })
+        const skills = await discoverSkills({ includeClaudeCodePaths: false })
         const skill = skills.find(s => s.name === "direct-format")
 
         // #then