feat(files): inspect, read, and download Slack files (#161)

Adds the `files` command group: `info`, `read`, and `download`, accepting a file ID or a Slack file permalink, canvas URL, or private files.slack.com URL.

`read` prefers Slack's `plain_text` for email files and refuses to print binary content; `download` streams the original bytes and will not overwrite an existing path. `fetchFile` follows redirects manually so browser cookies and bearer tokens are never forwarded to a non-Slack download host.

Closes #157