fix(mcp): prevent phantom memory URL edits

[phernandez]

Why

edit_note could reinterpret an unresolved memory URL route as a path in the active project. For append and prepend, that fallback could create a phantom note. A cross-project resolver match could also flow into a patch scoped to the wrong project and expose an internal entity ID in the resulting error.

What changed

• retain project ownership metadata from entity resolution
• stop cross-project matches before mutation and return an actionable project_id retry
• reject unresolved memory URL routes after the normal active-project compatibility lookup misses
• preserve valid existing memory URL path edits
• return structured JSON errors for both safe-stop cases
• add regressions proving no phantom file is created and no cross-project patch is attempted

Verification

• just fast-check
• just fast-test
• uv run pytest -q --no-cov tests/mcp/clients/test_clients.py tests/mcp/test_tool_edit_note.py tests/mcp/test_project_context.py (166 passed)
• just doctor

Risk

The routing behavior changes only after a memory URL project prefix fails to resolve and the identifier also misses in the selected project. Existing indexed memory URL paths continue through the prior compatibility fallback.

Closes #1066