Add CLI root and test labeler subcommand stub
Implements Tasks 3 and 4: root clap Parser in src/cli.rs dispatching to
Command enum in src/commands.rs, with Test variant nesting TestCmd in
src/commands/test.rs, which holds Labeler(LabelerCmd) in
src/commands/test/labeler.rs. LabelerCmd accepts target, --did, and
--subscribe-timeout arguments with humantime-based duration parsing and
1-second floor validation. Stub run() prints parsed arguments and returns
OK without implementation. Verification: cargo check and cargo build pass.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>