storkit: create 437_bug_strip_prefix_ci_panics_on_multi_byte_utf_8_input

.storkit/work/1_backlog/437_bug_strip_prefix_ci_panics_on_multi_byte_utf_8_input.md

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+---
+name: "strip_prefix_ci panics on multi-byte UTF-8 input"
+---
+
+# Bug 437: strip_prefix_ci panics on multi-byte UTF-8 input
+
+## Description
+
+The `strip_prefix_ci` function in `server/src/chat/transport/matrix/assign.rs` slices the input string at `prefix.len()` bytes without checking that the offset is a valid UTF-8 char boundary. When the input message starts with multi-byte characters (e.g. `⏺` which is 3 bytes), the slice can land mid-character, causing a panic.
+
+## How to Reproduce
+
+Send a Matrix message to the bot that starts with a multi-byte UTF-8 character (e.g. `⏺ storkit - wizard_confirm`) where the bot name byte length falls inside a multi-byte character.
+
+## Actual Result
+
+Thread panics: `byte index 6 is not a char boundary; it is inside '⏺' (bytes 4..7)`
+
+## Expected Result
+
+The function should return `None` (no match) without panicking, since an ASCII bot name cannot match a slice containing multi-byte characters.
+
+## Acceptance Criteria
+
+- [ ] strip_prefix_ci checks is_char_boundary before slicing
+- [ ] No panic when input contains multi-byte UTF-8 characters at the prefix boundary