docs(l10n): add AI agent instructions for updating po/XX.po files

Add a new section in the AI Agents chapter to provide clear instructions
for updating language-specific PO files. The improved documentation
significantly reduces both model interaction rounds and execution time.

Performance evaluation using qwen model:

    git-po-helper agent-test --runs=5 --agent=qwen update-po

Benchmark results (5-run average):

                    Before      After       Improvement
    Turns:          18          7           -61%
    Exec time:      42s         18s         -57%

    Turn range:     5-24        3-13
    Time range:     13s-65s     11s-26s

The optimization makes agent workflows more efficient and reduces
API costs by minimizing unnecessary LLM interactions and file content
checks.

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

Author: jiangxin

po/README.md

@@ -480,6 +480,21 @@ When asked to "update po/git.pot" or similar requests:
 The command will handle all necessary steps including file creation or
 update automatically.
 
+### Updating po/XX.po
+
+When asked to "update po/XX.po" or similar requests (where XX is a
+language code):
+
+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** the updated file after execution. Simply run the
+   command and consider the task complete.
+
+The command will handle all necessary steps including generating
+"po/git.pot" and merging new translatable strings into "po/XX.po"
+automatically.
+
 
 [git-po-helper/README]: https://github.com/git-l10n/git-po-helper#readme
 [Documentation/SubmittingPatches]: Documentation/SubmittingPatches