Feature: Extensible core memory file registry

[chubes4]

Problem

Core memory files (SOUL.md, USER.md, MEMORY.md) are hardcoded in two places:

1. MemoryFileRegistry in bootstrap.php — registers the three files at priorities 10, 20, 30
2. MemoryFilesSelector / PipelineMemoryFiles React component — hardcodes EXCLUDED_FILES = ['SOUL.md', 'USER.md', 'MEMORY.md'] to hide them from the picker

This means plugins can't register their own core memory files that get treated as first-class citizens (always injected, excluded from the optional picker).

Proposed

Make the core memory file registry dynamic so plugins can register additional always-on memory files:

PHP side

• MemoryFileRegistry::register() already exists and works — plugins can call it today
• Expose a filter or action at a well-known hook so plugins register their files cleanly
• Add a method like MemoryFileRegistry::get_core_filenames() that returns the list of always-injected files

JS/React side

• Instead of hardcoding EXCLUDED_FILES, fetch the core file list from the REST API (or from a localized script variable)
• New endpoint or extend existing: GET /files/agent could return { files: [...], core_files: ['SOUL.md', ...] }
• MemoryFilesSelector excludes whatever the backend says is core, not a hardcoded array

Use cases

• A plugin registers PLUGIN_CONTEXT.md as a core memory file at priority 25
• It always injects into AI calls, appears in the "core" section, and is excluded from the pipeline/flow picker
• If the plugin is deactivated, the file stops injecting automatically

Related

• Feature: Flow-scoped memory files #471 (flow-scoped memory files) — introduced the shared MemoryFilesSelector with the hardcoded exclusion
• Core directive: CoreMemoryFilesDirective (priority 20)
• Registry: MemoryFileRegistry in inc/Engine/AI/MemoryFileRegistry.php