feat: add shell completion support

[y-cg]

This PR adds shell completion support for multiple shells. Close #109.

Summary by CodeRabbit

• New Features

  • Shell completion added for Bash, Zsh, Fish, PowerShell, and Elvish to enable tab completion for commands and options.

• Documentation

  • README updated with a Shell Completion guide showing how to enable and persist tab completion.

• Packaging

  • Installers now include/install shell completion scripts so completions are available after installation.

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[coderabbitai]

📝 Walkthrough

Walkthrough

A new shell completion feature is introduced using the clap_complete crate. Users can run lumen completion <shell> to generate completion scripts for bash, zsh, fish, powershell, and elvish. Implementation includes CLI schema updates, a new completion module, README docs, and integration in the main entry point.

Changes

Cohort / File(s)	Summary
Dependency Management Cargo.toml	Added clap_complete = "4.4" to enable generating shell completion scripts.
CLI Schema & Subcommand src/config/cli.rs	Added CompletionShell enum (Bash, Zsh, Fish, PowerShell, Elvish) and a new Completion { shell: CompletionShell } subcommand. Added ValueHint annotations to several CLI fields (e.g., config, api_key, model, reference, query, pr, file, theme).
Completion Module src/command/completion.rs, src/command/mod.rs	New completion module with pub fn generate_completions(shell: CompletionShell) mapping CompletionShell to clap_complete::Shell and emitting completion scripts to stdout. Module exported via pub mod completion;.
Application Routing src/main.rs	Added early-return handling for the Completion subcommand: call generate_completions() and return Ok(()) before normal config/build/command dispatch.
Documentation README.md	Added "Shell Completion" section and TOC entry describing how to generate and persist completion scripts for Bash and Zsh.
Nix Packaging flake.nix	Reworked outputs to per-system structure via flake-utils.lib.eachDefaultSystem, added per-system package build (with postInstall installing completion scripts for bash/fish/zsh), and adjusted devShells packaging layout.

Sequence Diagram

sequenceDiagram
    participant User
    participant CLI as Clap CLI Parser
    participant Main as src/main.rs
    participant CompMod as completion module
    participant Stdout as Shell Stdout

    User->>CLI: lumen completion bash
    CLI->>Main: Route Completion{shell: Bash}
    Main->>CompMod: generate_completions(Bash)
    CompMod->>CompMod: Build CLI schema (Cli::command())
    CompMod->>CompMod: Map Bash → clap_complete::Shell
    CompMod->>Stdout: Generate & emit completion script
    Stdout->>User: Bash completion script

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Estimated Code Review Effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 With whiskers aquiver and a joyful bound,
Shell completions at last are found!
Bash, Zsh, Fish—tab completion's dream,
Lumen's completionist golden beam ✨
Scripts hop to stdout, our magic complete!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title 'feat: add shell completion support' directly and clearly summarizes the main change, matching the primary objective of adding shell completion functionality.
Linked Issues check	✅ Passed	The pull request successfully implements the requirements from issue #109: it adds a lumen completion <shell> generator supporting bash, zsh, fish, powershell, and elvish shells.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing shell completion support. The flake.nix restructuring is minimal and only includes logic to install completions during package build.

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