refactor(core): remove Vulture-confirmed dead code

[phernandez]

Why

Vulture found several production helpers and one complete MCP formatting module
that have no callers. Keeping these paths makes the codebase harder to navigate
and leaves obsolete compatibility layers looking like supported behavior.

This PR removes only findings that were confirmed unused in this repository and
cross-checked against the current Basic Memory Cloud source.

What Changed

• Deleted the unreferenced basic_memory.mcp.formatting module.
• Removed an unused cross-project move guidance formatter.
• Removed four unused private semantic-search repository wrappers and their
  now-unused import.
• Removed the unused internal type_filter argument from recent-activity
  formatting and updated its tests.

There are no public API, MCP schema, persistence, or runtime behavior changes.
The net change is 270 deleted lines across five files.

Implementation Details

• Ran Vulture at both broad and 100% confidence, then verified every selected
  symbol with exact repository searches.
• Kept Vulture findings that are required framework callback parameters for
  Alembic, Pydantic, and SQLAlchemy.
• Kept locally unused runtime protocol symbols that are imported by Basic
  Memory Cloud.
• Used Vulture through uvx; this PR does not add a project dependency or CI
  requirement.

Testing

Automated

• source .venv/bin/activate && python -m pytest -q tests/mcp/test_tool_recent_activity.py tests/mcp/test_tool_move_note.py tests/repository/test_semantic_search_base.py tests/repository/test_semantic_vector_sync.py tests/repository/test_hybrid_fusion.py tests/repository/test_vector_pagination.py tests/repository/test_vector_threshold.py: 108 passed.
• source .venv/bin/activate && ruff check src/basic_memory/mcp/tools/move_note.py src/basic_memory/mcp/tools/recent_activity.py src/basic_memory/repository/search_repository_base.py tests/mcp/test_tool_recent_activity.py: passed.
• source .venv/bin/activate && ruff format --check src/basic_memory/mcp/tools/move_note.py src/basic_memory/mcp/tools/recent_activity.py src/basic_memory/repository/search_repository_base.py tests/mcp/test_tool_recent_activity.py: passed.
• just fast-check: passed.
• just fast-test: expanded to the full suite after testmon detected an
  environment change; 4,675 passed and 41 skipped. The sole failure was the
  live postgres-openai semantic benchmark because OpenAI returned HTTP 429
  insufficient_quota.
• git diff --check: passed.

Manual

• Re-ran Vulture after the cleanup. The remaining four 100%-confidence
  production findings are required framework callback parameters.
• Confirmed exact searches return no references to the deleted module or
  helpers.

Risks / Follow-ups

• Risk is low because the change removes private, unreferenced paths without
  replacing behavior.
• The live OpenAI benchmark can be rerun when quota is available; local
  SQLite, Postgres, and FastEmbed coverage passed.
• No follow-up code changes are required.