Persistent share links are an undocumented out-of-band input #23

A share_link code source is cached on its URL alone (fetch_code only takes the code source JSON). That's correct for ordinary share links, which are immutable snapshots like https://arcade.makecode.com/84426-33815-03715-00484.

MakeCode also has persistent share links for signed-in users. They start with S and keep the same URL when the owner re-shares with "Update existing share link for this project" (pxt share docs, announcement). After the owner updates the project, a rebuild returns the old cached game until --full-rebuild, just like a git branch.

Relevant code: inputs/code.py, lines 69–76,`` the --full-rebuild help text in cli.py, lines 37–44, and the out-of-band list in docs/PROJECT_CONTEXT.md, line 254.

Relying on --full-rebuild for out-of-band sources is the documented design. The gap is that persistent share links aren't listed among those sources: the help text only mentions "the game's source files, icon, or git branch", so users have no reason to expect a share link to go stale.

Expected behavior: the --full-rebuild help text and the documented out-of-band list include persistent (S…) share links. Optionally, fetch_code could log a hint when a share link's ID starts with S.

Found while reviewing PR #7, toolchain at ce12c48.