Completion for option values (e.g., `--format={rich,tree,yaml,json}`)

[holmboe]

Enable shell completion for option values.

Already implemented:

• --format now uses OutputFormat enum with values: rich, tree, yaml, json
  • Shell completion shows available values when pressing Tab
  • Invalid values are rejected with helpful error message
  • strict is accepted as hidden alias for yaml (backwards compatibility)

Still needed:

• --ascii and --hyperlink: always, auto, never
• --log-level: DEBUG, INFO, WARNING, ERROR, CRITICAL

These could use enums similar to OutputFormat to enable shell completion.