feat(slack): org admin setting + emoji picker for ack emoji (#571)

* feat(slack): let orgs override the bot's ack emoji set via SLACK_ACK_EMOJI

An org-layer env knob (comma/space-separated Slack short-names, colons
optional, e.g. "mirai_thinking, mirai_ok") that fully replaces the stock
curated/default candidates for both the quick fallback reaction and the
model-picked ack emoji. Unset or empty keeps today's behavior.

Verified live: dev instance with SLACK_ACK_EMOJI=galaxy_brain reacted
:galaxy_brain: to an incoming DM before the reply.

Co-Authored-By: QM <noreply@ycombinator.com>

* test: use generic emoji names in ack-emoji fixtures

* style: prettier

Auto-fixed by watch-pipeline

* fix: address review findings — skip emoji.list refresh when an org override is set; disallow apostrophe in ack emoji names

* feat(slack): move ack emoji override from SLACK_ACK_EMOJI env to an org admin setting with emoji picker

* fix(lint): read Slack env bot token at the boundary, not process.env

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Co-authored-by: Josh France <josh@ycombinator.com>
Co-authored-by: QM <noreply@ycombinator.com>