nebula v5 plugin for deepseek harness

Key features

• Connect / authenticate against a graphd (nebula_connect), execute GQL on the same server-side session (nebula_execute), and close it (nebula_disconnect).
• Add connection presets — each with an alias (letters/digits/_/-, ≤ 64 chars), host, port, user, passwordRef, TLS mode, per-instance CA / client cert / key / servername, timeoutMs, and a note.
• Protocol — the vendored protos under src/proto/nebula/ are the official NebulaGraph 5.0 definitions (graph/common/vector) from nebula-go v5.
• Named instance profiles (Settings → NebulaGraph): manage any number of connection presets in the DSH settings page — each with its own host/port/user/passwordRef/TLS policy — addressed by a short alias.
• The model then just says "connect to the prod Nebula" and calls nebula_connect(instance: "prod")