feat: make eval_mode configurable via ENVECTOR_EVAL_MODE env var

[heeyeon01]

Problem

vault_core.py hardcodes eval_mode="rmp", which makes it impossible to connect Rune-Vault to envector-msa >= 1.4.0 which uses MM (Multi-Multiplication) mode. RMP and MM keys are not interchangeable, so the server rejects registration.

Solution

Read eval mode from ENVECTOR_EVAL_MODE environment variable (default: rmp for backward compatibility with envector.io cloud / 1.2.x).

Changes

• vault/vault_core.py — replace hardcoded "rmp" with EVAL_MODE module-level constant
• vault/docker-compose.yml — expose ENVECTOR_EVAL_MODE variable
• vault/.env.example — document the new variable with RMP/MM guidance

Notes

Changing this value requires deleting existing keys (vault_keys/) and restarting Vault so new mode-specific keys are generated.