Dispatchers are now always registered at compile time, removing the
`with_dispatcher` / `with_dispatchers` methods and the
`PathfinderConfig` API. The `dispatch_tree` feature now only controls
the matching strategy (trie vs linear list).

Add detailed doc comments, examples, and usage notes across
the completion module including ShellContext, Suggest, and
related traits.

Mark all source files with a comment indicating they are not optimized
for documentation purposes.

Replace bool-based settings with enums, refine visibility and
signatures, and standardize `Self` usage across the crate.

- Introduce `ErrorOutput`, `RenderOutput`, `PanicSilence`, `Verbosity`,
  `ColorOutput`, and `ProgressOutput` enums for clearer configuration
  semantics
- Make `GlobalResources`, `ProgramCell`, and
  `split_input`/`split_input_string` public
- Change hook info parameters to accept references and
  `split_input_string` to take `&str`
- Apply `Self` shorthand throughout implementations and add
  `#[must_use]` attributes where appropriate
- Enable strict clippy lints with targeted allowances

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.

Fix command tree matching when global flags precede the subcommand.