Shared quoting helper for path hints in bregctl and evidencectl

The next-step line of `bregctl init`, the build diagnosis hint of `evidencectl`, and the scaffold's next-step hint interpolate a project path unquoted, so a path containing whitespace is not copy-pastable.

Add one quoting helper and use it for every hint, so the hints change together. Cover each hint with a unit test whose path contains a space.

Origin: review of PR #850; declined there so the three hints move in one change.

Locate the implementations and existing unit tests for the next-step hints in bregctl init, evidencectl build diagnosis, and the scaffold. Start by comparing how each hint currently interpolates its project path. Done means all three use the shared quoting behavior and each has a unit test covering a path containing whitespace.

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