l10n: docs: add AGENTS.md with update POT instructions

l10n: docs: add AGENTS.md with update POT instructions

Add a new documentation file po/AGENTS.md that provides agent-specific
instructions for generating or updating po/git.pot, separating them
from the general po/README.md. This separation allows for more targeted
optimization of AI agent workflows.

Performance evaluation with the Qwen model:

    # Before: No agent-specific instructions; use po/README.md for
    # reference.
    git-po-helper agent-test --runs=5 --agent=qwen update-pot \
	--prompt="Update po/git.pot according to po/README.md"

    # Phase 1: add the instructions to po/README.md; the prompt
    # references po/README.md during execution
    git-po-helper agent-test --runs=5 --agent=qwen update-pot \
	--prompt="Update po/git.pot according to po/README.md"

    # Phase 2: add the instructions to po/AGENTS.md; use the built-in
    # prompt that references po/AGENTS.md during execution
    git-po-helper agent-test --runs=5 --agent=qwen update-pot

Benchmark results (5-run average):

Phase 1 - Optimizing po/README.md:

    | Metric      | Before  | Phase 1 | Improvement |
    |-------------|---------|---------|-------------|
    | Turns       | 17      | 5       | -71%        |
    | Exec. time  | 34s     | 14s     | -59%        |
    | Turn range  | 3-36    | 3-7     |             |
    | Time range  | 10s-59s | 9s-19s  |             |

Phase 2 - Adding po/AGENTS.md (further optimization):

    | Metric      | Before  | Phase 2 | Improvement |
    |-------------|---------|---------|-------------|
    | Turns       | 17      | 3       | -82%        |
    | Exec. time  | 34s     | 8s      | -76%        |
    | Turn range  | 3-36    | 3-3     |             |
    | Time range  | 10s-59s | 6s-9s   |             |

Separating agent-specific instructions into AGENTS.md provides:

- More focused and concise instructions for AI agents
- Cleaner README.md for human readers
- An additional 11% reduction in turns and 17% reduction in execution
  time
- More consistent behavior (turn range reduced from 3-7 to 3-3)

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