Replace individual memory directives with filter-based CoreMemoryFilesDirective

[chubes4]

Summary

Replaces the individual AgentSoulDirective and AgentMemoryDirective with a single CoreMemoryFilesDirective backed by a WordPress filter registry. Adds USER.md as a new default memory file. Removes deletion protection — all memory files are equal.

Before → After

BEFORE (one class per file):
Priority 20 → AgentSoulDirective     → reads SOUL.md
Priority 30 → AgentMemoryDirective   → reads MEMORY.md

AFTER (single directive, filter registry):
Priority 20 → CoreMemoryFilesDirective
               ├─ internal 10: SOUL.md
               ├─ internal 20: USER.md  (new)
               └─ internal 30: MEMORY.md

Design

• datamachine_memory_files filter — the single registry for all core memory files
• Files declare file (filename) and priority (sort order)
• Plugins/themes hook in to add, remove, or reorder files
• Adding a new memory file = one filter callback, not a new PHP class
• No core vs custom distinction — all memory files are just files

What changed

File	Change
CoreMemoryFilesDirective.php	New — single directive + default file registration via filter
AgentSoulDirective.php	Deleted
AgentMemoryDirective.php	Deleted
bootstrap.php	1 require instead of 2
AgentMemoryMigration.php	Adds USER.md template + ensure method, version bump to 1.2
FileAbilities.php	Removed deletion protection for SOUL.md/MEMORY.md
3 other directives	Updated priority order comments

Extensibility example

// A theme adding a brand voice file:
add_filter('datamachine_memory_files', function ($files) {
    $files[] = ['file' => 'BRAND-VOICE.md', 'priority' => 15];
    return $files;
});

Net: -164 lines, +214 lines (includes new USER.md template + ensure method). Removes 2 classes, adds 1.

[chubes4]

Clean refactor. Two classes → one directive + MemoryFileRegistry with filter-driven extensibility. USER.md as a default is smart.

One issue: AgentMemoryMigration.php is deleted entirely. That handles first-run creation of SOUL.md and MEMORY.md for new installs. Without it, a fresh DM install gets no default files on disk. MemoryFileRegistry::register() just registers filenames — it doesn't create files. You need a bootstrap/ensure step somewhere, even if simpler than the old migration class.

Also: the AgentPing::register() line in data-machine.php is missing indentation and leading backslash — looks like bleed from #309, not this PR.