fix(notifications): split Discord issue batches over 2,000 characters

[Furox-Art]

Closes #359

Problem

notifyDiscord() joins every new issue for a repository into a single content string and sends it with no length check. Discord rejects webhook payloads over 2,000 characters, and a batch of ~30 issues with ordinary-length titles already exceeds that. A rejected send also escapes the sequential repository loop in main(), so later repositories are not notified in that run.

Fix

• New helper lib/utils/splitDiscordContent.ts: splits content into chunks of at most 2,000 characters.
  • Lines are kept whole whenever they fit, so issue entries stay readable.
  • A single line that cannot fit is split on a boundary that never breaks a UTF-16 surrogate pair.
  • Joining the chunks with \n reproduces the input, so no notification is silently dropped.
• notifyDiscord() now sends the chunks sequentially, preserving allowedMentions: { parse: [] } (no link previews) and the existing failure semantics — a transport failure still propagates, with no automatic retry.

Scope is deliberately narrow: main()'s error handling is unchanged.

Validation

• New tests/test-discord-content-splitting.test.ts — 8 cases: empty content, below limit, exactly at the limit, one over, the 30-issue batch from the issue (every entry preserved, every chunk within the limit), a surrogate pair at the boundary, a single over-long line, and multiline content.
• Full suite: 65 passed / 0 failed (bun 1.4.2).
• tsc --noEmit clean, biome format clean on the touched files.

Disclosure: this PR was prepared with AI assistance (implementation and tests), reviewed and validated locally.

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