v0.15.1 — doctor sources-only suppression and add --source files→sources migration

Fixed

• doctor no longer warns "directory is empty" on sources-only stores. A
  sources-only store (the v0.14 fan-in consumer shape) has an intentionally
  empty store dir — every file it links lives elsewhere in the repo, declared
  via sources. The empty-store check keyed off directory geometry, so it
  fired a false-positive warning whose hint ("add files or remove the store")
  pointed at the wrong remediation. It now suppresses when the store (or any
  named target) declares sources, and the hint for a genuinely empty store
  mentions sources as a third option.
• add --source now migrates an existing files entry in one atomic write.
  Adding a sources entry for a target name that was previously a files
  entry used to error with "sources key is also listed in files; remove one" —
  the only path was hand-editing state.toml or a remove + add --source
  round-trip that briefly unlinks the live target link. add --source now
  drops the matching files entry and inserts the sources entry in a single
  state.toml write (the files→sources fan-in conversion the v0.14 plan
  envisioned), prints a "migrated from files entry" notice, and reports
  migrated_from_files: true in JSON. The filesystem link is untouched; the
  next apply repoints it.