fix: adopt start rejects plain local paths and calls file:// a test scheme

[runewright]

rune adopt start /absolute/path/to/rule.md fails with:

fatal: unsupported URL '/Users/.../ReviewMarkers.md': use https:// or file:// for tests

Two problems:

• A plain absolute or relative path is the natural spelling for adopting a local artifact and should resolve without a scheme.
• The message says file:// is "for tests", but local adoption is a production flow (the ReviewMarkers rule was adopted from a local harness deployment today via file://). The wording steers users away from the supported path.

Session: claude-code:1f95a181-3259-4c69-88fc-ffcfb21154f7