v0.5.0

First user-facing organization primitive: tag-based file labeling.
Backend-only release; desktop UI lands in v0.5.1.

Added

• Tag domain type + normalization (perima-core).
  Tag { id, name, first_seen } value type; normalize() applies
  trim → NFC → lowercase with MAX_TAG_LEN = 64 guard.
  CoreError::InvalidTag variant for validation failures.
  TagRepository trait with 8 methods (upsert_tag, delete_tag,
  attach, detach, list_tags, tags_for_hashes,
  files_with_tag, count_files_for_tag).
• V005 migration + SqliteTagRepository (perima-db).
  tags and file_tags tables (CRDT-compliant: soft deletes,
  updated_at + device_id on every mutable row, no UNIQUE on
  mutable columns, no FK cascades). Content-addressed via
  blake3_hash. Composite (blake3_hash, tag_id) covering index +
  reverse tag_id index. tags_for_hashes batches via
  params_from_iter; short-circuits on empty slice.
  14 new tests including Barrier-driven concurrent upsert.
• CLI tag subcommand (perima).
  perima tag add <path> <tags...> (1+ required),
  perima tag rm <path> <tag>,
  perima tag ls [--json] with per-tag file counts.
  perima ls --tag <name> filters the file listing to tagged files.
  3 new integration tests.