Slack connector: add Read/Write support for Slack Lists

The Slack connector currently supports Canvas read/write (Read Slack Canvas Content, Create Slack Canvas Document, Update Slack Canvas Document) but has no equivalent for Slack Lists.

Slack Lists are a distinct file type (file IDs like F08K7SF8VU0, URLs at slack.com/lists/...). The current slack_read_canvas tool returns file_not_found when called with a List file ID — it doesn't fall back or fail gracefully.

Requested tools:

• slack_read_list — read the items/rows of a Slack List by file ID
• slack_update_list — add, update, or check off items in a Slack List by file ID

Use case: Our team uses a Slack List as a deploy train tracker. The deploy assistant skill needs to read which PRs are queued and mark them as deployed post-ship. Without List support, both operations are manual workarounds.

Current behavior: slack_read_canvas("F08K7SF8VU0") → execution_failed: file_not_found

Expected behavior: A dedicated slack_read_list tool that returns list items, and slack_update_list for mutations.