Auto-select a sole discovered target

[V3RON]

Target-required commands now auto-select the only discoverable target instead of failing with a manual selection error. Users only need to run target list and target select when more than one target is available.

Summary

• add a CLI helper that checks daemon status and auto-selects a target when default discovery finds exactly one result
• route target-required commands through that helper while leaving explicit target-management and stored-artifact commands unchanged
• add CLI tests for single-target auto-selection, selected-target passthrough, and the multi-target guidance message

Closes #9.

Backward-Compatibility

• Existing commands, flags, daemon IPC, and persisted selected-target behavior remain compatible.
• target list, target select, target clear, start, stop, and status are unchanged.
• The only default-behavior change is that target-required commands now auto-select when discovery returns exactly one target; when multiple targets are available, they still stop and require explicit selection.

Risks

• Commands that previously surfaced No target selected may now surface discovery or auto-selection errors first when no target has been chosen yet.
• The helper depends on the same default discovery URLs as target list, so environments with transient extra targets may now hit the multi-target guidance path more often.
• If a command was missed and still uses plain ensureDaemon(), its behavior would remain inconsistent with the rest of the target-required commands.

Manual testing

• Setup: run a single debuggable app or runtime so only one target is available on the default discovery URLs.
• Run agent-cdp target clear to remove any previous selection.
• Run a target-required command such as agent-cdp console list.
• Verify the command succeeds without an explicit target select step and that agent-cdp status shows the selected target.
• Start a second debuggable target on a default discovery URL.
• Run agent-cdp target clear, then rerun agent-cdp console list.
• Verify the command stops with Multiple targets available. Run 'agent-cdp target list' and 'agent-cdp target select <id>'.