Add HC agent selection logic and handlers (hc_doctor_fix, hc_run_demo)

[binrogithub]

Motivation

• Provide deterministic selection of an HC agent when multiple candidates are available.
• Allow automated filling of missing demo values in non-interactive runs so demos can proceed without user input.
• Support a configurable preference order (explicit/config, default-tag, active/newest, lexical) for agent choice.

Description

• Add new package entry hc_agent and hc_agent.core with doctor.py implementing select_agent that prefers an explicit argument or config, then agents tagged with hc-agent-default=true, then tie-breaks by ACTIVE candidates using newest created_at and finally lexical order.
• Add hc_agent.core.handlers with hc_doctor_fix to fill missing values from defaults and by selecting an agent via doctor.select_agent, and hc_run_demo which auto-invokes hc_doctor_fix when non_interactive is true.
• Register handlers in a HANDLERS dict and include small typing/annotation adjustments (Optional usage).

Testing

• No automated tests were run as part of this change.
• Changes were added and committed to the repository; no unit or integration test execution was performed.

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Codex Task