feat(cli): one-command shell completion install (completions --install)

[sunerpy]

Summary

Make shell completions actually usable in one command. codegraph completions <shell> previously only printed the script to stdout, leaving users to hand-wire their rc/profile — which on PowerShell is actively broken (clap_complete emits using namespace ... at the top, so appending it into a non-empty $PROFILE fails with UsingMustBeAtStartOfScript, and re-appending duplicates it).

• feat(cli): completions <shell> --install — writes the completion script to the shell's standard location and reports the path. Idempotent (safe to re-run). Without --install, behavior is unchanged (stdout).
  • bash → ${XDG_DATA_HOME:-~/.local/share}/bash-completion/completions/codegraph (auto-loaded, no .bashrc edit)
  • zsh → ~/.zfunc/_codegraph (+ advises fpath+=~/.zfunc)
  • fish → ~/.config/fish/completions/codegraph.fish
  • powershell → %LOCALAPPDATA%\codegraph\completion.ps1 (a separate file so using namespace is legal at file start) + a single idempotent dot-source line in $PROFILE
  • elvish → ~/.config/codegraph/completion.elv (+ advises eval)
  • Design: generate a completion file + add one source/dot-source reference to the rc — never paste the full script inline; re-running deduplicates.
• docs: docs/cli.md expanded with per-shell enablement (--install + manual fallback), the PowerShell using/UsingMustBeAtStartOfScript rationale, and the Ctrl+Space / Set-PSReadLineKeyHandler -Key Tab -Function MenuComplete tip; concise zh-CN mirror; one-line README pointer.

Verification

• cargo test -p codegraph-rs green incl. new completions_powershell_install_is_idempotent + completions_bash_install_writes_file.
• Hands-on: PowerShell --install ×3 → exactly one dot-source line in $PROFILE; bash --install ×2 → one completion file, .bashrc untouched.
• cargo clippy --workspace --all-targets -- -D warnings clean; cargo fmt --all --check + make fmt clean; bash scripts/guardrail.sh exit 0.
• Windows x86_64 cross-compile (cargo-xwin) green.
• release-please will compute v0.5.0 (feat present).

Note

CI may flake on two pre-existing parallel tests (codegraph-watch::sync::tests::unchanged_file_hash_is_not_reindexed, codegraph-mcp::golden_mcp::tools_list_matches_upstream_names_and_schemas) — re-run if so; not introduced here.