[Issue]: feat(installer): Add Phase 4.5 agent copy with upgrade tracking

[WilliamBerryiii]

Issue Description

The HVE Core installer agent needs a new Phase 4.5 to support copying agents into target repositories. VS Code does not support external path settings for agents - they must reside within the workspace .github/agents/ directory. This creates a requirement for the installer to offer agent copying as part of the installation workflow.

Requirements

• Add Phase 4.5 between Phase 4 validation and success report
• Offer three selection options: Install all agents (recommended), Install RPI Core only, or Skip
• RPI Core bundle includes: task-researcher, task-planner, task-implementor, rpi-agent
• Implement collision detection for existing agent files with Overwrite/Keep/Compare options
• Create .hve-tracking.json manifest for Cruft-style upgrade tracking
• Support future upgrade workflows with diff-based prompts for modified files

Acceptance Criteria

• Phase 4.5 added to installer agent with agent selection prompt
• Collision detection prompts before overwriting existing files
• .hve-tracking.json manifest created with source version, SHA256 hashes, and timestamps
• Cross-platform copy operations (PowerShell + Bash)
• Upgrade workflow documented for future implementation

Additional Context

• Research document: .copilot-tracking/research/20260113-agent-copy-installer-research.md
• Current installer: .github/agents/hve-core-installer.agent.md
• 17 chatmodes + 1 agent available for copying
• Only 2 platform-specific agents (ADO/GitHub) - not worth separate selection question