feat(completions): add safe user-level install

[cxymds]

Summary

• add optional --install support to rc completions
• choose shell-specific user completion paths with XDG-aware defaults
• preserve existing scripts unless the user explicitly passes --force
• install through a temporary file in the destination directory for atomic replacement
• document default paths and custom --install-dir usage

Why

Completion generation already exists, but users must manually discover the correct location and install the generated script. This implements the completion-install slice of rustfs/backlog#1382 while keeping stdout generation unchanged.

User impact

Users can run rc completions <shell> --install. Existing files are never overwritten implicitly; --force is required to replace them.

BREAKING change assessment

BREAKING: no. This only adds optional flags; the existing rc completions <shell> stdout behavior is unchanged. The marker is included because this PR updates the protected command reference documentation.

Validation

• cargo fmt --check
• cargo test -p rustfs-cli commands::completions::tests (7 tests in each CLI target)
• cargo clippy -p rustfs-cli --all-targets -- -D warnings
• command smoke test covering first install, refused overwrite, and forced replacement

Closes rustfs/backlog#1484
Refs rustfs/backlog#1382