fix(core): mandate backticks for file references in system prompt

- update system prompt to explicitly require backticks around file:// URIs
- add examples distinguishing correct backticked format from incorrect ones
- ensures file references are consistently formatted for UI and parsing

Author: aweis89

lua/opencode/core.lua

@@ -170,12 +170,13 @@ M.send_message = Promise.async(function(prompt, opts)
   params.system = [[
 # Code References
 
-**CRITICAL: Always use the file:// URI scheme when referencing files in responses.**
+**CRITICAL: Always use the file:// URI scheme when referencing files in responses AND wrap them in backticks.**
 
 Format: `file://path/to/file.lua`, `file://path/to/file.lua:42`, or `file://path/to/file.lua:42-50`
 
 Examples:
-- CORRECT: "The error is in file://src/services/process.ts:712"
+- CORRECT: "The error is in `file://src/services/process.ts:712`"
+- INCORRECT: "The error is in file://src/services/process.ts:712"
 - INCORRECT: "The error is in src/services/process.ts:712"
 
 This matches the file:// URI format that the reference picker already parses from your responses, enabling automatic navigation.