new plumbing for the upcoming flow: git_fetch_to_tag (pulls a
sha from proposed into applied and pins it as a tag in one
shot), git_rev_parse (normalises shas + reads back tag
targets), git_tag / git_tag_annotated (lightweight vs body-
carrying for failed/denied), git_read_tree_reset (replace
working tree without moving HEAD — lets main stay on last
known-good across an in-flight build), git_update_ref (ff main
on deploy). annotated tag bodies go via stdin to avoid escape
games. all dead-code-allowed; callers land in subsequent
commits.
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