fix(agent): show duration units in debugger timeout flags

[bcherry]

lk agent debugger start --help printed --idle-timeout stop. The usage string put backticks around stop, and urfave/cli treats a backticked word as the flag's value placeholder. Neither --idle-timeout nor say --timeout said which units the value takes.

Before:

--idle-timeout stop  Stop the session after this long without any command (0 to keep it running until stop) (default: 30m0s)

After:

--idle-timeout DURATION  Stop the session after DURATION without any command, such as 30m, 2h, or 90s (0 keeps it running until stop) (default: 30m0s)
--timeout DURATION       Give up waiting for the agent's reply after DURATION, such as 2m or 90s (default: 2m0s)

Help text only, so no behavior changes. go vet passes, and the debugger unit tests pass.

Found in docs review on livekit/web#6338.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JLeMSp9LH6VsBNFjm8cg6h

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