Add help text for global flags, commands, and subcommands in
help.txt. Add metadata descriptions for linter and metadata
commands.

Enable the dispatch_tree and comp features to generate
completion scripts during build and register global help
and completion support in the main entry point.

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.