Expand CLI completion with context-aware value suggestions

[rmanibus]

Summary

Shell completion should offer context-aware values when the command expects a known identifier or enum, instead of only static command and flag names.

Initial scope

• complete snapshot refs for commands like restore, ls, forget, and diff
• complete store refs where a configured store name is expected
• complete object keys for cat where feasible
• complete other config-backed names from profiles.yaml where relevant

Notes

• PR Add dynamic completion for profile and auth refs #201 introduces the first dynamic completion path for -profile and -auth-ref
• follow-up work should reuse the same hidden completion query mechanism instead of embedding shell-specific logic per value type
• completions should stay non-interactive and fail quietly when config or repository access is unavailable

Acceptance criteria

• bash, zsh, and fish all surface dynamic candidates for the supported contexts
• completions honor -profiles-file when config-backed names are queried
• repository-backed completions avoid noisy errors when the repo is unavailable or locked
• tests cover both candidate generation and emitted shell script hooks