feat(shell): add xonsh integration

[EzraCerpac]

PR Description

Adds xonsh shell integration for tv init xonsh.

This includes:

• xonsh shell detection and CLI enum support
• prompt-toolkit keybindings for smart autocomplete and shell history
• xonsh setup docs for direct init and saved integration files
• tests for init output, unsupported standalone completions, keybindings, metadata, and script rendering

Standalone clap completions remain unsupported for xonsh, matching nu/cmd-style unsupported behavior.

Parts of this PR were generated with AI assistance. I reviewed the changes and checked them locally.

Checklist

• my commits and PR title follow the conventional commits format
• if this is a new feature, I have added tests to consolidate the feature and prevent regressions
• I have added a reasonable amount of documentation to the code where appropriate

[Copilot]

Pull request overview

Adds xonsh support to Television’s shell integration (tv init xonsh) by extending shell detection/CLI enums, introducing a new xonsh integration script using prompt-toolkit keybindings, updating documentation, and adding test coverage for init/completions behavior.

Changes:

• Add xonsh as a supported shell target for tv init, including keybinding rendering and metadata handling.
• Introduce a new xonsh integration script (completion.xsh) with prompt-toolkit keybindings for smart autocomplete and shell history.
• Update user/developer docs and add CLI tests for xonsh init output and “unsupported completions” behavior.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/cli/special.rs	Adds CLI-level tests covering tv init xonsh output markers and tv completions xonsh unsupported behavior.
television/utils/shell/completion.xsh	New xonsh prompt-toolkit integration script implementing smart autocomplete + shell history bindings.
television/utils/shell.rs	Extends shell enum/mapping/template rendering to include xonsh, plus unit tests for keybindings/metadata/script rendering.
television/cli/args.rs	Adds Xonsh to the CLI shell ValueEnum so tv init xonsh / tv completions xonsh parse correctly.
docs/user-guide/04-shell-integration.md	Documents how to enable xonsh integration via direct init and via a saved integration file.
docs/developers/shell-integration-local.md	Updates local development instructions to include xonsh.

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[cfbender]

tested and working for me on tv 0.15.6 and xonsh 0.23.6!

thanks!