fix(core): update guidance for file reference formatting

- enhance clarity in the instructions for referencing files
- emphasize the use of the file:// URI scheme as critical
- provide examples to illustrate correct and incorrect formats
- ensure compatibility with the existing reference picker for navigation

Author: aweis89

lua/opencode/core.lua

@@ -167,8 +167,19 @@ M.send_message = Promise.async(function(prompt, opts)
     state.current_mode = opts.agent
   end
 
-  params.system =
-    'When referencing files in your responses, always use the file:// URI scheme (e.g., file://path/to/file.lua, file://path/to/file.lua:42 for a specific line, or file://path/to/file.lua:42-50 for a range). This helps with navigation and tooling integration.'
+  params.system = [[
+# Code References
+
+**CRITICAL: Always use the file:// URI scheme when referencing files in responses.**
+
+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"
+- 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.
+]]
 
   params.parts = context.format_message(prompt, opts.context)
   M.before_run(opts)