docs: collect the documentation changes that wait for the SSH setup screen

Part of #2917.

This issue is the one list of changes to documentation/ that wait for the SSH setup screen (#2917). Until #2917 ships, users cannot set up a remote instance in the UI, so the docs do not describe remote instances. Netlify builds getqui.com from main, so the PR for this issue merges in the same release as the #2917 PR, not earlier.

When a rollout PR changes what a remote instance does and the docs must describe it, add an item here with the PR number. Do not change the docs for remote instances in that PR.

Changes

• documentation/docs/features/torrent-management.md: Say that folder cleanup also runs on a remote instance that can write (#3318). Say that orphan scan cannot delete on a remote instance until #3236, so a folder that cleanup leaves stays in place. From @com6056's review of #3318.
• documentation/docs/features/orphan-scan.md: Add back the peer notes and the "Instances reached over SSH" section. The wording is at ee61359f. Moved here from #2917 (from #2968).
• documentation/docs/features/automations.md, "Free Space source": Add back the SSH wording that #2966 removed. Moved here from #2917.
• documentation/docs/intro.md:18: Reword it, because "remote instance" there means an instance without file access. Moved here from a comment on #2917 (2026-10-19).
• documentation/docs/features/cross-seed/hardlink-mode.md:22: The sentence that a remote instance cannot compare file identity, so any existing target stops the add there, becomes: a target that is already qui's own link to the same file is reused on an instance with local filesystem access and on a remote instance where qui can run commands on the host; on a remote instance where qui only has SFTP, it cannot tell its own link from another file, so any existing target stops the add there. Capability wording, not tier wording. From #3322.
• AGENTS.md: Delete the exception that sends docs changes for remote instances to this issue.