feat(i18n): add Italian (it) locale

[Johell1NS]

Description of changes

Adds a complete Italian (it) locale — all 532 keys, following the same shape as the existing locales:

• New src/i18n/it.ts typed against I18nDictionary (missing/extra keys fail compilation), so key parity with en.ts is enforced at build time.
• Registered in LOCALE_DEFINITIONS (code: "it", label: "Italiano", dateLocale: "it-IT"), in alphabetical position so the setup-wizard numbering for existing users is unchanged.
• Selectable via BOT_LOCALE=it or the setup wizard.

Italian is a major Telegram market and one of the most-requested locales, so this closes a real gap.

Translation approach

• Translated from en.ts; used fr.ts / pt.ts as reference for house tone and button phrasing.
• Every {placeholder}, emoji and \n escape preserved exactly (verified programmatically, see below).
• Button labels kept short so they still fit on mobile.
• Terms Italian users normally keep in English are left untranslated: OpenCode, worktree, token, prompt, MCP, cron, bash. Likewise detached HEAD, which is identical in practice.
• Consistent informal "tu" register throughout, matching the original English tone.

How it was tested

Gate	Result
npm run build (tsc)	clean
npm run lint (eslint src tests --max-warnings=0)	clean
npm run typecheck	clean
npm test	1419/1419 passed (138 files)

The I18nDictionary type is the meaningful gate for key parity. I additionally verified programmatically that the key list is identical to en.ts (532 keys, none missing, none extra) and that every {placeholder} token matches its English counterpart one-for-one.

README.md (all three spots: Languages: line, Supported locales: list, BOT_LOCALE table row) and .env.example (# Supported locales: comment) were updated per docs/LOCALIZATION_GUIDE.md.

Closes issue (optional)

• n/a

Checklist

• PR title follows Conventional Commits: <type>(<scope>)?: <description>
• This PR contains one logically complete change
• Branch is rebased on the latest main
• I ran npm run lint, npm run build, and npm test
• If this PR is OS-sensitive, behavior/limitations for Linux/macOS/Windows are described

[grinev]

@Johell1NS thanks for contribution!