agentdb_*-delete MCP tools (alpha.8)

Three new MCP tools wired through agentdb@3.0.0-alpha.13's native Cypher-routed delete API:

• agentdb_hierarchical-delete — calls ReflexionMemory.deleteEpisode (graph + vector + SQL all-in-one)
• agentdb_causal-edge-delete — calls GraphDatabaseAdapter.deleteEdgesByEndpoints(from, to, relation?) (Cypher-injection-safe)
• agentdb_causal-node-delete — calls GraphDatabaseAdapter.deleteNode(id, {cascade: true}) returns native {deletedNode, deletedEdges} audit

All wrapped in MutationGuard (fail-closed) + AttestationLog (audit). Unblocks /adr-index re-index when ADR files are deleted from disk — stale nodes + dangling supersedes / amends / related / depends-on edges are now scrubbable. Closed #1784.