Gist names percent-encode sync paths, and empty files are absent

GitHub gists are flat and text-only: a file name cannot contain /, and a file cannot hold empty content (create fails with HTTP 422; on update, GitHub deletes a file set to ""). The Gist Backend therefore stores each home-relative path under its percent-encoded name, leaving only [A-Za-z0-9._-] literal, so .pi/agent/settings.json travels as .pi%2Fagent%2Fsettings.json. The mapping is one-to-one and decoded on fetch, so paths survive the round trip while root files like AGENTS.md stay human-readable in the GitHub UI. Zero-byte files are skipped at collection time with a warning and are absent from the Snapshot: an empty file has nothing to carry, and truncating a file to empty reads as a deletion in the three-way merge.

Considered options: a delimiter token for / (collides with real names, not reversible); a path-to-name mapping stored in the manifest (fetch must derive the path from the name alone, before any config is trusted); a placeholder for empty content (breaks the content hash and shows up in the GitHub UI).