BREAKING CHANGE: `#[completion]` functions must now take `ShellContext`
by value instead of `&ShellContext`. Reference parameters are reserved
for resource injection.

Allow completion functions to omit the context parameter, accept owned
ShellContext or any From<&ShellContext> type, and return any type
implementing Into<Suggest>. Add From<&str> for SuggestItem and derive
Clone for ShellContext.

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

Refactor #[chain], #[renderer], #[help], and #[completion] macros to
keep
the original user function as a named item instead of inlining its body.
Trait method implementations now call the original function by name,
injecting resources from the application context.

This improves debugging (stack traces reference user function names),
error messages, and macro expansion clarity.

Restructure flat module layout into logical directories (attr/, derive/,
func/, systems/, extensions/, utils.rs). Tighten internal function
visibility to pub(crate). All public API signatures remain unchanged.