refactor: switch to clap with subcommands for CLI parsing

Replace hand-rolled arg parser with clap derive. Restructure as
subcommands: import, cmd, script. Import subcommand supports
--category, --measure, --time, --skip, --extract, --axis, --formula,
--name, --no-wizard, and --output flags for configurable imports.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Cargo.toml

@@ -22,6 +22,7 @@ flate2 = "1"
 unicode-width = "0.2"
 dirs = "5"
 csv = "1"
+clap = { version = "4.6.0", features = ["derive"] }
 
 [dev-dependencies]
 proptest = "1"