Add channel thread listing with pagination and filtering

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Problem

Today tw can list inbox threads and filter that list by channel (tw inbox --channel ...), but there is no direct way to list threads for a specific channel as a first-class command. That makes it hard to:

• browse all threads in a channel (not only inbox-centric views)
• paginate through channel history
• apply channel-scoped filters in a predictable way for humans and agents

Proposal

Add channel-thread listing support with pagination and filtering.

Possible UX options (choose one and document in skill content):

1. tw channel threads <channel-ref> [workspace-ref]
2. tw thread list --channel <channel-ref> [workspace-ref]

Minimum Requirements

• Channel reference resolution should support existing ref patterns (numeric ID, id:, Twist URL where applicable, and fuzzy name when unambiguous).
• Pagination support:
  • --limit <n>
  • --cursor <cursor> (or equivalent continuation token)
  • machine-readable next cursor in --json and --ndjson output
• Filtering support (initial set):
  • unread only (--unread)
  • date window (--since, --until)
  • archived/include-archived behavior (if API supports it)
• Output parity with existing conventions:
  • human-readable text output
  • --json, --ndjson, and --full

Acceptance Criteria

• User can list threads for a single channel without going through inbox.
• Results can be paginated reliably across multiple pages.
• Filters are applied server-side where supported; otherwise clearly documented fallback behavior.
• Tests cover ref resolution, pagination metadata, and filter behavior.
• src/lib/skills/content.ts is updated to include the new command + examples.

Notes

This aligns with recent CLI ergonomics in related Doist CLIs (URL/ref-friendly navigation plus explicit view/list routes) and would close a current discoverability gap in tw.