Flatten indented config

A CI pipeline file uses a tiny indentation format. Every non-blank line is indented by exactly 2 spaces per nesting level and contains a ":". A line whose text after the colon is empty, like "build:", opens a nested block; a line like "image: node:20" is a leaf whose value is everything after the FIRST colon, trimmed (values are never empty, and may themselves contain colons). Blank lines can appear anywhere and mean nothing. A block with no children contributes nothing. The document is well-formed. Flatten it into a dict mapping the dot-joined path of nested keys to each leaf value.