l10n: docs: add update PO instructions in AGENTS.md

Add a new section to po/AGENTS.md to provide clear instructions for
updating language-specific PO files. The improved documentation
significantly reduces both conversation turns and execution time.

Performance evaluation with the Qwen model:

    # Before: instructions in po/README.md; the custom prompt
    # references po/README.md during execution
    git-po-helper agent-test --runs=5 --agent=qwen update-po \
        --prompt="Update po/zh_CN.po according to po/README.md"

    # After: instructions in po/AGENTS.md; the built-in prompt
    # references po/AGENTS.md during execution
    git-po-helper agent-test --runs=5 --agent=qwen update-po

Benchmark results (5-run average):

    | Metric      | Before  | After  | Improvement |
    |-------------|---------|--------|-------------|
    | Turns       | 22      | 4      | -82%        |
    | Exec. time  | 38s     | 9s     | -76%        |
    | Turn range  | 17-39   | 3-9    |             |
    | Time range  | 25s-68s | 7s-14s |             |

Signed-off-by: Jiang Xin <worldhello.net@gmail.com>

Author: jiangxin

po/AGENTS.md

@@ -59,6 +59,17 @@ When asked to generate or update `po/git.pot` (or the like):
    command and consider the task complete.
 
 
+### Task 2: Updating po/XX.po
+
+When asked to update `po/XX.po` (or the like):
+
+1. **Directly execute** the command `make po-update PO_FILE=po/XX.po`
+   without reading or checking the file content beforehand.
+
+2. **Do not verify, translate, or review** the updated file after execution.
+   Simply run the command and consider the task complete.
+
+
 ## Human translators remain in control
 
 Git translation is human-driven; language team leaders and contributors are