feat(direct): support prefix and regex default args

[risu729]

Summary

direct.default_args currently matches exact shim command names only. Add a future matcher model that can apply defaults by prefix, including subcommand-like invocations, or by regex.

Motivation

Some direct commands have families of invocations where the right biwa run defaults are shared by more than one exact shim name. For example, commands that never depend on synchronized project files could consistently get --skip-sync without listing every concrete command key.

Notes

• Keep the current exact-key behavior as the simple/default path.
• Design precedence explicitly when exact, prefix, and regex matchers overlap.
• Avoid making static shim installation enumerate unbounded regex matches.

Follow-up from #632.