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>

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