v0.7.0 — --only write scoping (spec 010)

Added

• --only FILE / --only-from FILE — scope writes to specific files (feature 010). darnlink now
  separates the two scopes the positional path used to fuse: the index scope (which files are
  read — still the whole tree, so a link's target resolves wherever it lives) and the write scope
  (which files are modified). --only narrows the latter; --only-from reads the list from a file or
  stdin (-), so a caller can pipe git diff --cached --name-only in without darnlink learning about
  git. This makes the common "anchor the links in the file I'm committing, touch nothing else" case
  possible — previously you either scanned your subtree (and the tool couldn't see out-of-subtree
  targets, so it anchored nothing) or ran repo-wide (and rewrote everyone's links).
• --no-target-writes — with --only, refuse the one write that otherwise lands outside the
  scope (adding a uuid to a target so a link can be anchored). Links that would need it stay plain
  and are reported; the guarantee becomes absolute: no file outside --only is touched.
• New finding kinds in human and --json output: out_of_scope, target_uuid_write,
  target_write_refused. --json gains write_scope and suppressed_outside_write_scope.

Fixed

• out_of_scope no longer misreported as no_frontmatter. A plain link whose target exists but
  was never scanned (outside path, or excluded) used to be reported as "target has no frontmatter"
  — stating as fact something the run never checked. It now has its own kind and an honest message.
  This is the confusion that motivated feature 010.