feat: add shell completion support (bash, zsh, fish)

[Wirasm]

Summary

• Add kild completions <shell> subcommand that generates shell tab-completion scripts using clap_complete
• Supports bash, zsh, fish, powershell, and elvish
• Completions are generated dynamically from the clap CLI definition, so they stay in sync as commands change

Usage

# Generate and install completions
kild completions bash > ~/.kild-completions.bash
kild completions zsh > ~/.kild-completions.zsh
kild completions fish > ~/.config/fish/completions/kild.fish

Test plan

• cargo fmt --check passes
• cargo clippy --all -- -D warnings passes
• cargo test --all passes (all 162 tests)
• cargo build --all succeeds
• kild completions bash outputs valid bash completion script
• kild completions zsh outputs valid zsh completion script
• kild completions fish outputs valid fish completion script
• kild completions (no arg) errors with usage hint
• kild completions invalid errors with valid shell options

Closes #89

[Wirasm]

PR Review Summary

Critical Issues (1 found)

Agent	Issue	Location
silent-failure-hunter	clap_complete::generate() writes to stdout without error handling. If stdout fails (broken pipe, disk full, redirect to write-protected path), user gets no error and thinks completions were generated. Add std::io::Write + stdout().flush()? after generate call.	crates/kild/src/commands/completions.rs:14

Suggestions (2 found)

Agent	Suggestion	Location
silent-failure-hunter	The .ok_or("Shell argument is required")? path is unreachable (clap enforces required args). Consider .expect("shell arg guaranteed by clap") to match intent, though current pattern is consistent with other handlers.	crates/kild/src/commands/completions.rs:9
pr-test-analyzer	Optional: add integration test verifying kild completions bash produces non-empty output to stdout. Current CLI parsing tests are adequate for the thin handler pattern.	crates/kild/tests/cli_output.rs (new)

Strengths

• Clean, minimal implementation following KISS/YAGNI — thin handler delegates to clap_complete
• Proper workspace dependency management (clap_complete in root Cargo.toml)
• Good test coverage: 3 CLI parsing tests covering valid, missing, and invalid shell args
• Bonus cleanup: moved unused imports in health.rs to test-only scope
• Follows established command handler patterns consistently

Documentation Updates

• CLAUDE.md — Added kild completions command examples for all 5 shells (commit 3d1f694)

Verdict

NEEDS FIX — One stdout flush issue to address before merge.

Recommended Actions

1. Add use std::io::Write; and std::io::stdout().flush()?; after the generate call
2. Consider the .expect() suggestion (optional — current pattern is consistent)
3. Integration test for output is nice-to-have, not blocking