feat(coding-agent): ResourceLoader, package management, and /reload command (#645)

- Add ResourceLoader interface and DefaultResourceLoader implementation
- Add PackageManager for npm/git extension sources with install/remove/update
- Add session.reload() and session.bindExtensions() APIs
- Add /reload command in interactive mode
- Add CLI flags: --skill, --theme, --prompt-template, --no-themes, --no-prompt-templates
- Add pi install/remove/update commands for extension management
- Refactor settings.json to use arrays for skills, prompts, themes
- Remove legacy SkillsSettings source flags and filters
- Update SDK examples and documentation for ResourceLoader pattern
- Add theme registration and loadThemeFromPath for dynamic themes
- Add getShellEnv to include bin dir in PATH for bash commands

packages/coding-agent/docs/extensions.md

@@ -115,10 +115,22 @@ Additional paths via `settings.json`:
 
 ```json
 {
-  "extensions": ["/path/to/extension.ts", "/path/to/extension/dir"]
+  "extensions": [
+    "npm:@foo/bar@1.0.0",
+    "git:github.com/user/repo@v1",
+    "/path/to/extension.ts",
+    "/path/to/extension/dir"
+  ]
 }
 ```
 
+Use `pi install` and `pi remove` to manage extension sources in settings:
+
+```bash
+pi install npm:@foo/bar@1.0.0
+pi remove npm:@foo/bar
+```
+
 **Discovery rules:**
 
 1. **Direct files:** `extensions/*.ts` or `*.js` → loaded directly
@@ -146,13 +158,17 @@ Additional paths via `settings.json`:
     "zod": "^3.0.0"
   },
   "pi": {
-    "extensions": ["./src/safety-gates.ts", "./src/custom-tools.ts"]
+    "extensions": ["./src/safety-gates.ts", "./src/custom-tools.ts"],
+    "skills": ["./skills/"],
+    "prompts": ["./prompts/"],
+    "themes": ["./themes/dark.json"]
   }
 }
 ```
 
 The `package.json` approach enables:
 - Multiple extensions from one package
+- Skills, prompts, and themes declared alongside extensions
 - Third-party npm dependencies (resolved via jiti)
 - Nested source structure (no depth limit within the package)
 - Deployment to and installation from npm