feat(src/bridge/discord): prefix handle with @ and collapse case-equal names

[phorcys420]

Tweaks how the Discord -> Linear bridge attributes authors in authorOf (src/bridge/discord/map.ts).

• Handle is now prefixed with @: Display (@handle) instead of Display (handle).
• The display-name/handle equality check is case-insensitive, so when they only differ by case the handle is dropped and just the display name is shown. e.g. Phorcys (display) + phorcys (handle) now renders as Phorcys instead of Phorcys (phorcys). The display-name casing is preserved.

Verified with bun format, bun lint, and tsc --noEmit (all clean). No test suite in this repo.

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This PR was generated by Coder Agents on behalf of @phorcys420.