AGENTS.md: add quick code location technique (#26569)

Author: kbkpbot

AGENTS.md

@@ -581,6 +581,22 @@ skipped coverage in the summary.
 * JS/native/wasm: prefer small, focused test files with
   `-b js|native|wasm` and `-show-c-output` where applicable; avoid
   broad `-b <backend> test vlib/`.
+* Quick code location technique:
+  When debugging code generation issues, add unique comment tags to
+  relevant code generation points (e.g., `/*tom51*/`) in the compiler
+  source. Rebuild and generate C code, then search the generated file for these
+  tags to quickly map generated C code back to the exact compiler source
+  location. This is especially useful when multiple code paths generate
+  similar-looking output and you need to identify which path is actually used.
+  Example:
+  ```v
+  // In vlib/v/gen/c/assign.v:
+  g.write('builtin___option_ok/*tom51*/(&(${styp}[]) { ')
+
+  // After rebuild, search generated C code for "tom51":
+  // builtin___option_ok/*tom51*/(&(int[]) { ... });
+  ```
+  Remember to remove these debug tags after fixing the issue.
 
 ## Compiler Architecture
 The V compiler has the following stages, orchestrated by the
@@ -708,4 +724,4 @@ v.callgraph, etc.
   * New tests or test infra changes.
   * Output tests or `.out` updates.
   * Broad refactors or compiler-wide changes.
-  * Unsure which slow test runner applies to your change.
+  * Unsure which slow test runner applies to your change.