Replace hardcoded hai run default with live agent picker

[abonneth]

Note

Low Risk
CLI and documentation behavior only; scripts that relied on the implicit default must pass --agent.

Overview
hai run no longer defaults to a fixed agent. The --agent / -a flag is optional in an interactive TTY: the CLI lists agents from the live catalog and prompts for a number. In non-interactive runs (--json or no TTY), omitting --agent fails with a message to pass --agent or use hai agents list.

Request size validation (assert_request_under_limit) now runs before agent selection or any catalog API call, so oversized tasks still fail without listing agents.

Docs and the hai-agents MCP skill examples switch the showcased agent from h/web-surfer-holo3-1-35b to h/web-surfer-pro. CLI tests pass -a explicitly and add coverage for the missing-agent error and validate-before-picker ordering.

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