feat: add watch command for Socket Mode event streaming

[co42]

Summary

• Add slackline watch command — connects via Slack Socket Mode (WebSocket) and streams events as JSONL to stdout
• Normalizes 14 event types (message, dm, reaction_added/removed, member_joined/left, file_shared, status_changed, channel_created/deleted/archive/unarchive/rename, team_join) into a consistent output format
• Defaults to your channels: on startup, fetches your channel membership via users.conversations and filters events to those channels only
• --all-channels flag opts into workspace-wide events (Socket Mode delivers everything)
• Filtering: --events, --channels / --exclude-channels (names or IDs), --exclude-subtypes, --raw
• Enriches message_changed events with text from nested message field
• Enriches reaction events with the reacted-to message text via API fetch (cached)
• Lazy name cache resolves user/channel display names via API on first encounter
• token create simplified: composable --write and --watch flags (read-only by default, can combine both)
• token create --watch includes guided setup for the optional app-level token (xapp-) needed for Socket Mode
• Shared channel resolution: all commands now accept channel names (general, #general) or IDs (C1RCG46LS) via Client::resolve_channel() — resolves public, private, mpim, and im channels
• User name resolution: users info and users presence accept @name, name, or U... ID via Client::resolve_user()
• Richer human-readable output: channels/users/files info now shows ID, topic, purpose, email, timezone

Review fixes (second round)

Bugs fixed:

• Handle 6 missing event types: channel_created/deleted/archive/unarchive/rename, team_join — with channel/user name caching from payloads
• Fix user name resolution priority: display_name > real_name > name (matches users info display)
• Add W prefix to looks_like_id for Enterprise Grid shared channels
• Replace expect() with ? in push_events_handler (no more panic)
• Use ChannelNotFound error variant; batch resolve reports all missing names

Improvements:

• Move subtype filter to pre-filter (before expensive normalize_event API calls)
• Cache fetch_message_text results for reactions (avoids duplicate API calls)
• Add debug output alongside display in socket error logging
• Replace dead All match arm with unreachable!()
• Return None for nameless IM channels instead of caching "#"
• Users list/search use scroller pagination (was single-page)

Refactoring:

• Extract normalize_channel_user_event helper shared by 5 event types
• Extract user_display_name helper for consistent name priority

First round review fixes

• Early filtering: extract event type and channel ID cheaply from raw events before API calls
• Batch channel resolution: resolve_channels() resolves multiple names in a single paginated scan
• Arc state: immutable WatchState fields wrapped in Arc<[T]> to avoid per-event vec clones
• Client reuse: socket mode listener reuses the same SlackHyperClient
• Channel type fix: resolve_channel now includes private/mpim/im types
• Reaction dedup: extracted normalize_reaction helper

Usage

# Watch (defaults to your channels)
slackline watch                                            # Stream events (default: message,dm,reaction)
slackline watch --all-channels                             # All workspace channels
slackline watch --all-channels --exclude-channels noisy    # All except specific channels
slackline watch --events all                               # All event types
slackline watch --channels general,infra                   # Only these channels
slackline watch --exclude-channels moon-landing            # All except these
slackline watch --exclude-subtypes bot_message             # Skip bot messages
slackline watch --raw                                      # Raw slack-morphism JSON

# Token setup
slackline token create --watch                             # Setup: read + watch
slackline token create --write --watch                     # Setup: read + write + watch

# Channel/user names work everywhere
slackline channels info general                            # By name
slackline users info @peter                                # By @name, name, or ID

Test plan

• cargo build + cargo clippy clean (zero warnings)
• slackline watch --help shows all flags
• slackline token create --watch prints setup instructions
• Manual test: events stream as JSONL, Ctrl-C gracefully disconnects
• --exclude-channels / --exclude-subtypes filtering works
• Default watch only shows events from channels you are in
• Channel and user name resolution works in all commands
• slackline users list in workspace with >100 users (verify pagination)
• slackline users info @name resolves correctly

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This PR was created with assistance from Claude Code.