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.

Refactor `build_dispatch_body` to take a caller-provided `no_match`
fallback token stream instead of hardcoding `build_entry_fallback`
calls. Thread the fallback through recursive calls so child paths are
preferred over exact endpoints at the same depth, matching the dynamic
dispatcher's longest-prefix-wins behavior.

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.

Rework bash completion script to fix word-index tracking when the
cursor is in the middle of a word, compute the current word from
COMP_LINE truncated at COMP_POINT, and use space-separated option
arguments. Also trim colon prefixes from completion items when
COMP_WORDBREAKS includes colons.

Fix zsh completion script to escape colons in completion items
and descriptions, preventing parsing issues with `_describe`.
Also iterate over the original completions array for simple
completions to correctly extract completion items.

trait