l10n: AGENTS.md: add quotation mark preservation guidelines

l10n: AGENTS.md: add quotation mark preservation guidelines

Add a "Preserving Quotation Marks" section to prevent AI-assisted
translation and review from incorrectly converting language-specific
UTF-8 curly quotes (e.g., „ U+201E, " U+201C for Bulgarian) into
ASCII straight quotes " (U+0022), which would cause PO string
truncation and syntax errors.

Also update the "Special characters" item in the Quality checklist
to reference the new section.

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