Require explicit skill and agent targets for VS Code marketplace installs

[Railly]

Context

The rich VS Code Marketplace Install action sends only source and name. The handler defaults agents to an empty array and omits the skill selector. The shared installer interprets an empty agent list as every agent.

Evidence:

• vscode/src/panels.ts:262 sends the incomplete install message.
• vscode/src/rich-extension.ts:1085-1094 applies the defaults.
• src/marketplace.ts maps an empty agent list to the wildcard target.
• vscode/src/parity.ts:24-41 already contains an explicit target-selection flow.

Task

Require at least one explicit agent, carry the exact selected skill identifier, and make global or project scope explicit before installing.

Acceptance criteria

• The rich Marketplace UI cannot install without selecting a target.
• Only the selected skill is requested from multi-skill repositories.
• The command arguments contain only the selected agents and scope.
• Tests assert the exact install request for single and multiple targets.