refactor(pifs): remove dead semantic retrieval code

[BukeLy]

Summary

Removes dead PIFS semantic retrieval code left behind after converging semantic recall on browse.

Why

The current PIFS command surface routes semantic file discovery through browse. Keeping old semantic fallback APIs, semantic folder projection helpers, and hybrid aggregate search paths adds unused behavior to pre-release code that should stay minimal.

What Changed

• Removed empty command dispatch extension points and an unused stable-target helper from PIFSCommandExecutor
• Removed PageIndexFileSystem.search(..., semantic=True) fallback behavior and search_semantic_channel()
• Removed semantic folder projection APIs and the now-unused semantic_folder_policy.py
• Removed hybrid aggregate retrieval paths from the projection backend, keeping only explicit summary, entity, and relation channel search
• Updated tests to verify semantic recall through browse_semantic_files() instead of old search APIs
• Removed legacy metadata migration guard code for old unpublished field names

Verification

• /Users/chengjie/Projects/PageIndex/.venv/bin/python -m pytest tests -q -> 115 passed, 6 warnings
• git diff --check HEAD~1..HEAD
• Residual scan found no production/test references to removed dead-code symbols such as COMMAND_METHODS, search_semantic_channel, _semantic_search, apply_semantic_folder_projection, aggregate_hybrid_entity_relation, or semantic_folder_policy
• Old removed command strings only remain in tests that assert they are absent or rejected

Risk

Medium. This intentionally removes unpublished public helpers and semantic fallback paths. The active browse / add semantic paths remain covered by tests.