Add watch-position (start-from) commands

[altaywtf]

Context

The put.io iOS/tvOS rewrite's agent-readiness harness (putdotio/putio-ios#146) tests resume-playback flows, which need seeded watch positions. The CLI has no start-from/watch-position surface.

What this delivers

Read/set/reset of a file's start-from position (e.g. files start-from get|set|reset <file-id>), JSON output, --dry-run on writes.

Notes

The harness calls the API directly until this lands.