Update changelog and issue tracker notes for the enhanced
`default_completion` behavior.

When a concrete entry's custom completion handler produces a `Suggest`
value, combine it with the default subcommand suggestions instead of
replacing them entirely. `FileCompletion` still falls back to the
default completion since it cannot be meaningfully merged.

Add `Metadata<B>` trait and `#[metadata(Entry)]` attribute macro
to attach arbitrary, compile-time-typed metadata to entries.
Implement `ProgramCollect::get_metadata<T>()` for runtime retrieval,
backed by a global registry populated by `register_metadata!`.

Extend `pathf` with `MetadataPattern` to resolve metadata types
across modules at build time. Add two examples demonstrating
metadata usage with and without pathf integration.

Fix command tree matching when global flags precede the subcommand.

Add `#[completion(EntryFallback)]` syntax to enable custom
completion suggestions for unmatched entries, merging fallback
results with default completions via `Suggest::combine()`.