📝 Add configured logger file control API docs

docs/api/configured-logger-file-append-mode.md

@@ -0,0 +1,75 @@
+---
+name: configured-logger-file-append-mode
+group: api
+category: runtime
+update-time: 20260512
+description: Read the current append-mode policy used by the configured runtime file sink.
+key-word:
+    - logger
+    - runtime
+    - file
+    - public
+---
+
+## Configured-logger-file-append-mode
+
+Read the current append-mode policy used by a `ConfiguredLogger` file sink. This helper exposes whether future reopen behavior is currently append-oriented.
+
+### Interface
+
+```moonbit
+pub fn ConfiguredLogger::file_append_mode(self : ConfiguredLogger) -> Bool {}
+```
+
+#### input
+
+- `self : ConfiguredLogger` - Config-driven runtime logger whose append-mode policy should be inspected.
+
+#### output
+
+- `Bool` - Current append-mode policy for the file sink.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- File-backed sinks report their current append policy.
+- Queued file sinks forward the policy from the wrapped file sink.
+- Non-file sinks return `false`.
+- This helper reports runtime file policy, not whether a file is currently writable.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Runtime Append-policy Visibility
+
+When diagnostics should show how future reopen behavior is configured:
+```moonbit
+let append = logger.file_append_mode()
+```
+
+In this example, the configured logger exposes current reopen policy directly.
+
+#### When Validate Runtime Policy Changes
+
+When policy mutation should be observable after a setter call:
+```moonbit
+ignore(logger.file_set_append_mode(true))
+ignore(logger.file_append_mode())
+```
+
+In this example, callers verify the updated append-mode policy.
+
+### Error Case
+
+e.g.:
+- If the configured sink is not file-backed, the method returns `false`.
+
+- If callers need the full current file policy rather than just append mode, `file_policy()` is the better API.
+
+### Notes
+
+1. Use this helper when append policy is the only file setting you need to inspect.
+
+2. Pair it with reopen helpers when debugging runtime file behavior.