Document detectShell() API and private field compile warning

completers.md:
- Added Shell Detection section with AeshRuntimeRunner.detectShell()
  API, usage example, and detection order documentation

annotation-processor.md:
- Note that the processor emits a compile-time warning for private
  fields suggesting package-private visibility (PR #547)

Author: stalep

content/docs/aesh/annotation-processor.md

@@ -128,7 +128,7 @@ For each option, argument, and mixin field, the generated code provides:
 
 - **Public / package-private fields** -- The generated code uses direct field access (`((MyCommand) inst).myField = value`). No `java.lang.reflect.Field`, no `getDeclaredField()`, no `setAccessible()`. This is the fastest possible path and requires no GraalVM native-image reflection configuration.
 
-- **Private fields** -- The generated code resolves `Field` constants once in a `static {}` initializer and caches them for reuse. This avoids repeated class hierarchy walks but still uses `getDeclaredField()` + `setAccessible()` at class load time, and requires reflection entries in native-image configuration (which the processor generates automatically -- see [GraalVM Native Image](#graalvm-native-image) below).
+- **Private fields** -- The generated code resolves `Field` constants once in a `static {}` initializer and caches them for reuse. This avoids repeated class hierarchy walks but still uses `getDeclaredField()` + `setAccessible()` at class load time, and requires reflection entries in native-image configuration (which the processor generates automatically -- see [GraalVM Native Image](#graalvm-native-image) below). The processor emits a **compile-time warning** for private fields suggesting package-private visibility.
 
 ## Generated Code
 

content/docs/aesh/completers.md

@@ -373,3 +373,26 @@ String fishScript = ShellCompletionGenerator.generateDynamic(
 ShellCompletionGenerator generator = ShellCompletionGenerator.forShell(ShellType.ZSH);
 String script = generator.generate(parser, "myapp");
 ```
+
+### Shell Detection
+
+Use `AeshRuntimeRunner.detectShell()` to detect the current shell from environment variables. This is the same logic used by `--aesh-completion` and `--aesh-completion-install` for auto-detection.
+
+```java
+import org.aesh.AeshRuntimeRunner;
+import org.aesh.util.completer.ShellCompletionGenerator.ShellType;
+
+ShellType shell = AeshRuntimeRunner.detectShell();
+if (shell != null) {
+    System.out.println("Current shell: " + shell);
+} else {
+    System.out.println("Could not detect shell");
+}
+```
+
+Detection order:
+1. `PSModulePath` — PowerShell (checked first to avoid misdetection from bash wrapper scripts)
+2. `FISH_VERSION` — Fish
+3. `ZSH_VERSION` — Zsh
+4. `BASH_VERSION` — Bash
+5. `$SHELL` fallback — checks the path for fish/zsh/bash/pwsh