refactor!: redesign the reprex mode interface

[eitsupi]

Summary

• replace the independent reprex and auto-format booleans with off, on, and format modes
• reorganize reprex configuration under [startup] and [reprex]
• use explicit --reprex=<off|on|format> and :reprex on|off|format interfaces
• move reprex runtime state out of prompt configuration and make formatter selection typed across interactive and script execution paths
• derive formatter commands and unavailable-backend diagnostics from exhaustive backend metadata, with inline snapshots for each user-facing context
• report all removed reprex configuration keys in a single arf config check run
• update session information, banners, completions, generated schema, documentation, and regression tests

Breaking changes

The former [startup.mode] and [mode.reprex] tables, auto-format fields, --auto-format, :autoformat, and :format interfaces are removed without compatibility aliases. Removed configuration keys are detected before deserialization and reported together as validation errors instead of being silently ignored or revealed one at a time.

The new configuration shape is:

[startup]
reprex = "off"

[reprex]
comment = "#> "
formatter = "air"

[prompt.indicators]
reprex = "[reprex] "
reprex_format = "[format] "

Validation

• cargo fmt --check
• cargo test -p arf-console
• cargo clippy -p arf-console --all-targets -- -D warnings
• git diff --check