[gen740]

Summary

• Add cli::action::custom for one-off non-terminal action pipeline steps.
• Tighten pack::callback with a CallbackInvocable concept.
• Update the advanced example to use action::custom and add a full struct-based custom action example.
• Add a git-style command example inspired by clap's git.rs.
• Expand Actions & Pipelines docs and example pages for custom actions and git-style subcommands.

Validation

• nix run .#build
• ctest --test-dir build --output-on-failure

[Copilot]

Pull request overview

This PR expands cli20’s action pipeline capabilities by introducing cli::action::custom<fn> for inline/non-terminal pipeline steps, tightening callback action constraints, and improving Windows Unicode handling by moving UTF-16→UTF-8 conversion outside the parser. It also adds new example programs and documentation updates to demonstrate custom actions and git-style subcommands.

Changes:

• Add cli::action::custom<fn> and improve action validation/type deduction helpers in include/cli/action.hh.
• Introduce cli::utf16_to_utf8(argc, argv) and remove Windows wchar_t* argv parsing overloads in favor of explicit conversion.
• Add/refresh examples and docs: new git and custom_action examples plus expanded docs for defining custom pipeline steps.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
TODO.md	Marks validation/action-type work as completed; adds note for nargs::rest.
tests/test_parse.cc	Updates tests to use action::custom and explicit UTF-16→UTF-8 conversion for Windows-wide argv tests.
include/cli/parser.hh	Adds Utf8Argv + utf16_to_utf8; removes wchar_t* parse overloads and updates Windows guidance.
include/cli/argument.hh	Updates action validation type extraction to match tuple-based Action::validate() return.
include/cli/action.hh	Adds ok/fail/ignore helpers, strengthens action validation, introduces action::custom, and tightens pack::callback via a concept.
examples/unicode_echo.cc	Uses explicit UTF-16→UTF-8 conversion in wmain before calling UTF-8 parser path.
examples/git.cc	Adds a git-style, command-heavy example with nested subcommands.
examples/custom_action.cc	Adds a reusable struct-based custom conversion example.
examples/advanced.cc	Updates advanced example to use cli::action::custom<fn>.
docs/docs/tutorial.md	Links to custom pipeline step docs.
docs/docs/examples/index.mdx	Adds entries for Custom Action + Git examples and renumbers sidebar ordering.
docs/docs/examples/git.mdx	New documentation page for the git example.
docs/docs/examples/env-fallback.mdx	Renumbers sidebar position/title to account for new examples.
docs/docs/examples/distribution-and-platform.mdx	Renumbers sidebar position/title to account for new examples.
docs/docs/examples/custom-action.mdx	New documentation page for the custom action example.
docs/docs/examples/compiler.mdx	Renumbers sidebar position/title to account for new examples.
docs/docs/examples/advanced.mdx	Mentions action::custom and links to Custom Action example.
docs/docs/actions.mdx	Expands guidance and adds a concrete action::custom example for custom pipeline steps.

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