📝 Add record patch API docs

docs/api/prefix-message.md

@@ -0,0 +1,79 @@
+---
+name: prefix-message
+group: api
+category: patching
+update-time: 20260512
+description: Create a reusable record patch that prepends text to the message.
+key-word:
+    - patch
+    - message
+    - transform
+    - public
+---
+
+## Prefix-message
+
+Create a `RecordPatch` that prepends a fixed prefix to `rec.message`. This helper is useful for tagging records with stable context without changing every call site.
+
+### Interface
+
+```moonbit
+pub fn prefix_message(prefix : String) -> RecordPatch {}
+```
+
+#### input
+
+- `prefix : String` - Text inserted before the original message.
+
+#### output
+
+- `RecordPatch` - Patch that returns a record whose message starts with `prefix`.
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- The patch preserves the original message content after the inserted prefix.
+- It does not touch target, level, timestamp, or fields.
+- This helper is useful for environment tags, subsystem labels, and compatibility shims.
+- When combined with other patches, composition order controls whether the prefix is applied before or after other message rewrites.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Add A Stable Channel Marker
+
+When console output should show one visible label:
+```moonbit
+let logger = Logger::new(console_sink())
+  .with_patch(prefix_message("[worker] "))
+```
+
+In this example, every emitted message starts with `[worker] `.
+
+#### When Compose With Redaction
+
+When records need both labeling and field protection:
+```moonbit
+let patch = compose_patches([
+  prefix_message("[safe] "),
+  redact_field("token"),
+])
+```
+
+In this example, message text is rewritten while fields remain independently patchable.
+
+### Error Case
+
+e.g.:
+- If `prefix` is empty, the patch behaves like a pass-through message rewrite.
+
+- If several patches prepend text, the final message reflects composition order.
+
+### Notes
+
+1. Prefer this helper over concatenating prefixes at every log call site.
+
+2. Keep prefixes short so they do not drown out the original message text.
+