Feature request: Add user and channel lookup commands

[zomglings]

Problem

When using clacks send -u <username>, user resolution can fail due to:

1. Rate limiting on the users.list API endpoint
2. No way to look up or search for users/channels without sending a message

Currently, if you don't know the exact username, display name, or user ID, there's no way to find it through clacks itself.

Proposed Solution

Add two new subcommands:

clacks users

# List users (with optional search/filter)
clacks users list
clacks users list --filter "zi"
clacks users search <query>

# Lookup specific user
clacks users lookup <identifier>  # by ID, username, or email

clacks channels

# List channels
clacks channels list
clacks channels list --filter "general"

# Lookup specific channel
clacks channels lookup <identifier>  # by ID or name

Relevant Slack API Endpoints

The following endpoints would be useful:

• GET https://slack.com/api/users.list - List all users in workspace
• GET https://slack.com/api/conversations.list - List all channels
• Headers: Authorization: Bearer <token>

Example API call pattern (details obfuscated):

curl -s "https://slack.com/api/users.list?limit=500" \
  -H "Authorization: Bearer xoxp-XXXX-XXXX-XXXX-XXXX" \
  | jq -r '.members[] | select(.deleted == false) | "\(.id) \(.name) \(.real_name)"'

Additional Context

The resolve_user_id function in messaging/operations.py already calls users.list() internally, but:

1. It's not exposed as a CLI command
2. It doesn't support pagination (noted as TODO in the code)
3. Results aren't cached, leading to repeated API calls and rate limiting

Caching the user/channel list in the sqlite database (as mentioned in the TODO comments) would also help avoid rate limiting issues.