Preserve user content in agent config files on coldbox ai refresh

[Copilot]

coldbox ai refresh blindly overwrites agent config files (e.g. CLAUDE.md / AGENTS.md), destroying any user or AI-agent customizations on every run.

Approach

Introduce <!-- COLDBOX-CLI:START --> / <!-- COLDBOX-CLI:END --> demarcation markers in the generated files. On refresh, only the content between those markers is replaced; everything after the end marker is preserved.

Changes

Templates (agent-modern-instructions.md, agent-flat-instructions.md)

• Wrap all framework-managed content in COLDBOX-CLI:START / COLDBOX-CLI:END markers
• Move "Custom Application Details" user section to after the end marker with an explanatory comment
• Restructure modern template so all managed content is contiguous above the user section

AgentRegistry.cfc

• Add MANAGED_SECTION_START / MANAGED_SECTION_END static constants
• Add mergeUserContent(filePath, newContent) — extracts the user section from an existing file and re-appends it after the refreshed managed block:
  • No existing file → write as-is (first install)
  • Existing file without end marker → overwrite (old format, no user section to preserve)
  • Existing file with end marker → replace managed section, keep everything after it
• Wire mergeUserContent() into configureAgent() for all agent types (including Claude's AGENTS.md)

Example of resulting file structure:

<!-- COLDBOX-CLI:START -->
<!-- ⚡ Managed by ColdBox CLI — refreshed on `coldbox ai refresh`. Do not edit between markers. -->

# MyApp - AI Agent Instructions
... framework-generated content ...

<!-- COLDBOX-CLI:END -->

<!-- ℹ️ YOUR PROJECT DOCUMENTATION — ColdBox CLI will NOT overwrite this section. -->

## Custom Application Details
... user/AI-authored content survives refresh ...

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[Copilot]

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