memory: add POST /api/v2/memory/delete so everos-mcp forget_session works on self-hosted servers #483

Context

The official MCP server everos-mcp exposes a forget_session tool that calls POST /api/v2/memory/delete with {session_id, app_id, project_id} to delete everything one connection stored. Self-hosted EverOS has no such route, so the tool currently degrades to a message ("this deployment does not support deleting memories through the API"), verified against a local server on 2026-10-10. No crash, but self-hosted users cannot delete through MCP.

Cloud contract to align with

evermind-everos-cloud/docs/api.md §7 defines the endpoint:

• POST /api/v2/memory/delete, success 204 with no body
• filters: user_id XOR agent_id, session_id, sender_id; at least one of user_id / agent_id / session_id
• three-state semantics per field: omitted = skip the filter, null/"" = match empty, value = exact match

The same doc marks the Cloud implementation as a placeholder (returns 204, deletes nothing; semantics to be landed by the pipeline owner). So this is not yet a contract gap to close in a hurry; it is a feature both sides still have to define.

Open questions before implementing

1. What a session-scoped delete removes. Episodes under the session are the obvious part. Atomic facts, profile clusters and agent cases were derived from those episodes by the offline pipeline — delete them too, recompute them, or leave them? (everos-mcp documents that the profile and learned skills survive forget_session.)
2. Three stores, one delete. Markdown is the source of truth; SQLite state and LanceDB rows must follow. The cascade already handles deleted change events for single files; a batch delete should go through the same path rather than touch the index directly.
3. Filter semantics. Whether to implement the full three-state filter from day one or only the session_id (+ owner) shape that everos-mcp sends.

Suggested path

Wait for the Cloud pipeline owner to land the deletion semantics, then implement the same contract here (route + service + markdown delete + cascade propagation), and remove the "not supported" branch in everos-mcp's forget_session. Until then the MCP docs state the limitation.

Refs: EverOS-MCP src/everos_mcp/client.py::delete_session, src/everos_mcp/server.py::forget_session.