l10n: docs: add review instructions in AGENTS.md

Add a new "Reviewing po/XX.po" section to po/AGENTS.md that provides
comprehensive guidance for AI agents to review translation files.

Translation diffs lose context, especially for multi-line msgid and
msgstr entries. Some LLMs ignore context and cannot evaluate
translations accurately; others rely on scripts to search for context
in source files, making the review process time-consuming. To address
this, git-po-helper implements the compare subcommand, which extracts
new or modified translations with full context (complete msgid/msgstr
pairs), significantly improving review efficiency.

A limitation is that the extracted content lacks other
already-translated content for reference, which may affect terminology
consistency. This is mitigated by including a glossary in the PO file
header. git-po-helper-generated review files include the header entry
and glossary (if present) by default.

The review workflow leverages git-po-helper subcommands:

- git-po-helper compare: Extract new or changed entries between two
  versions of a PO file into a valid PO file for review. Supports
  multiple modes:

  * Compare HEAD with the working tree (local changes)
  * Compare a commit's parent with the commit (--commit)
  * Compare a commit with the working tree (--since)
  * Compare two arbitrary revisions (-r)

- git-po-helper msg-select: Split large review files into smaller
  batches by entry index range for manageable review sessions. Supports
  range formats like "-50" (first 50), "51-100", "101-" (to end).

Evaluation with the Qwen model:

    git-po-helper agent-run review --commit 2000abefba --agent qwen

Benchmark results:

    | Metric           | Value                            |
    |------------------|----------------------------------|
    | Turns            | 22                               |
    | Input tokens     | 537263                           |
    | Output tokens    | 4397                             |
    | API duration     | 167.84 s                         |
    | Review score     | 96/100                           |
    | Total entries    | 63                               |
    | With issues      | 4 (1 critical, 2 major, 1 minor) |

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

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