Slack custom emoji: two-way sync + saveCustomEmoji SDK

[KrisBraun]

Summary

Phase C of comprehensive Slack reaction sync — workspace custom emoji (:party_parrot:), both directions.

• Twister: new Integrations.saveCustomEmoji(emoji: NewCustomEmoji[]) so connectors can populate Plot's custom-emoji cache (changeset included).
• Slack connector:
  • emoji:read requested as an optional scope (degrades to standard-only if not granted).
  • emoji.list wrapper + syncCustomEmoji (on channel-enable + daily): fetches workspace emoji, resolves alias: chains, upserts as slack:<teamId>/<name> refs via saveCustomEmoji, caches the name set.
  • Inbound: an unmapped reaction whose name is a known workspace custom emoji becomes a slack:<teamId>/<name> ref (renders as an image in Plot) instead of being dropped.
  • Outbound: a slack:<teamId>/<name> ref is unwrapped to its name for reactions.add/remove.
  • reactionCapabilities.customEmoji flipped none → workspace.

Builds on Phase A (comprehensive standard emoji + skin tones) and Phase B (server-driven reaction capabilities).

Picker scoping + image rendering live in the core repo (separate change): the server stamps an opaque twist_instance.custom_emoji_scope, and the Flutter picker offers a connection's custom emoji from the synced custom_emoji cache. No client meta/provider logic.

Test Plan

• pnpm --filter @plotday/connector-slack test — 20 pass (incl. new syncCustomEmoji, inbound-ref, outbound-unwrap tests)
• pnpm exec tsc --noEmit + plot lint + pnpm build clean
• pnpm validate-changesets valid
• Runtime/app verification with a real Slack workspace (not yet done)

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