akb v0.20.0

Changed

• BREAKING: the knowledge-base state directory is renamed .akb/ → .agent-kb/,
  and its config file loses the redundant leading dot: .akb.yaml → akb.yaml.
  The directory now carries the project's name; the tool-facing files inside it
  keep the tool's. akb init creates the new layout, and every command —
  discovery, path resolution, read/write, lint, templates, search — recognizes
  only it. There is no compatibility shim: a base in the old layout is reported
  as not a knowledge base, with the expected marker path named. Migrate an
  existing base by renaming the directory and config file (mv .akb .agent-kb && mv .agent-kb/.akb.yaml .agent-kb/akb.yaml) and updating the .akb/search.db*
  line in the base's .gitignore. Tool-named surfaces are unchanged: the
  AKB_KB environment variable, the *.akb.bak backup extension, and the
  page.akb CEL map key.
• The on-disk layout now has one definition site. internal/path exports
  StateDirName, ConfigFileName, and the StateDir/ConfigPath/
  TemplatesDir/SearchDBPath helpers, replacing the ".akb" string literals
  scattered across the command and internal packages.