Simplify the history package

[dgageot]

Tidy up pkg/history so it's easier to read and maintain, without
changing any user-facing behaviour or the on-disk file format.

What changed

• load(): replaced the manual byte counting / line splitting /
  strconv.Unquote parser with bytes.Lines + json.Unmarshal,
  symmetric with how entries are written.
• Shared dedup helper: extracted addInMemory so Add and load
  share the "remove prior occurrence + append" logic instead of
  carrying two different implementations.
• LatestMatch: uses slices.Backward for cleaner backward
  iteration.
• Dropped the options pattern: New(opts ...Opt) + Opt +
  WithBaseDir + private options struct collapsed into a single
  New(baseDir string). Empty string falls back to the user's home
  directory.
• Dropped the current == -1 cursor sentinel: the cursor is now
  always a valid index in [0, len(Messages)], which kills a special
  case in Previous().
• SetCurrent clamps to [0, len(Messages)], eliminating latent
  out-of-bounds panics in Previous/Next if a caller ever passed an
  invalid index.
• Reordered the file so the public navigation/search API is at the top
  and the persistence/migration helpers are grouped at the bottom.
• Removed the unused json:"messages" struct tag (the file format is
  one JSON-encoded string per line, the struct itself is never
  marshalled).
• Tightened doc comments and added a test for SetCurrent with
  out-of-range values.