feat(setup): add Slack and iMessage channel flows (experimental)

Slack: interactive driver walks through app creation, validates the
bot token via auth.test, installs the adapter, and prints a
post-install checklist for the webhook URL + Event Subscriptions
config. No welcome DM since Slack needs a public URL before inbound
events work — the driver's own "finish in Slack" note replaces the
outro "check your DMs" banner.

iMessage: picks local (macOS) vs remote (Photon) mode. Local mode
opens the node binary's directory in Finder so the user can drag it
into Full Disk Access. Remote mode prompts for Photon URL + API key.
Asks for the operator's phone/email, then wires the first agent
including a welcome iMessage.

Both marked "(experimental)" in the askChannelChoice picker.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

setup/lib/claude-assist.ts

@@ -64,6 +64,10 @@ const STEP_FILES: Record<string, string[]> = {
   'telegram-validate': ['setup/channels/telegram.ts'],
   'pair-telegram': ['setup/pair-telegram.ts', 'setup/channels/telegram.ts'],
   'discord-install': ['setup/add-discord.sh', 'setup/channels/discord.ts'],
+  'slack-install': ['setup/add-slack.sh', 'setup/channels/slack.ts'],
+  'slack-validate': ['setup/channels/slack.ts'],
+  'imessage-install': ['setup/add-imessage.sh', 'setup/channels/imessage.ts'],
+  'imessage': ['setup/channels/imessage.ts'],
   'teams-install': ['setup/add-teams.sh', 'setup/channels/teams.ts'],
   'teams-manifest': ['setup/lib/teams-manifest.ts', 'setup/channels/teams.ts'],
   'init-first-agent': [