v0.8.1

Added

• Index auto-discovery: a search without --index finds the index at the
  searched prefix, at any parent prefix (scoped to the searched subtree),
  or at a location remembered from an earlier --index run.
• --files lists every indexed key for a target from the index alone,
  honoring -g, --key-prefix, --key-regex, and --since/--until.
• skills/seagrep/SKILL.md: an agent-facing skill covering discovery, the
  cheap-first workflow, the cost model, and turn economy.