feat(channel list): add --via-counts flag for Enterprise Grid

[cawilliamson]

What

channel list --via-counts — lists the current user's joined conversations (channels, MPIMs, DMs) with names on Enterprise Grid workspaces, where browser/session tokens can't use users.conversations/conversations.list (Slack returns enterprise_is_restricted).

Why

On Enterprise Grid, Slack restricts org-wide conversation enumeration (conversations.list, users.conversations) for browser/session (xoxc/xoxd) tokens — this is intentional on Slack's side and not something a CLI can route around. The web client builds its sidebar from client.counts, which is still permitted and returns every conversation the user has joined (split across channels/mpims/ims, ids only). This PR exposes that same path behind an explicit flag so Grid users can list their joined conversations from the CLI without an admin-approved app token.

--all (full workspace enumeration) remains inherently unavailable via browser tokens on Grid — only an app token (xoxp) with conversations:read can do that. --via-counts is therefore incompatible with --all and --user (client.counts is current-user, joined-only, and has no server-side pagination; --limit slices locally, --cursor is ignored).

How

• listConversationsViaCounts (src/slack/channels.ts): calls client.counts, collects ids from channels/mpims/ims, enriches each via conversations.info (parallelised with Promise.all), and returns the same ConversationsPage shape as the other list helpers. Keeps { id } if conversations.info fails for a given conversation.
• channel list (src/cli/channel-command.ts) gains --via-counts; mutually exclusive with --all/--user.

Testing

• bun run typecheck — clean
• bun run lint — no new warnings/errors (10 pre-existing warnings unchanged)
• bun run format:check — clean
• bun test — 242 pass, 0 fail (3 new tests for listConversationsViaCounts: enumeration + enrichment, info-failure fallback, limit slicing / id-less entries)

Example

$ agent-slack channel list --via-counts
{
  "channels": [
    { "id": "C…", "name": "general", "is_channel": true, … },
    { "id": "G…", "name": "group-dm-name", "is_mpim": true, … },
    { "id": "D…", "name": "alice", "is_im": true, … }
  ]
}