[Agent] Implement runtime configuration mechanism (CLI args & Env vars)

[varev-dev]

Goal

Provide a consistent runtime configuration layer for Volta components (agent, server, CLI) that supports CLI arguments + environment variables with deterministic precedence, validation, and clear startup diagnostics.

Acceptance criteria

• Runtime options can be configured via both env vars and CLI flags.
• Precedence works exactly as: flag > env > default for overlapping keys.
• Invalid runtime input fails fast with clear, actionable error messages.
• --help output documents all runtime flags; docs include env-var mapping.
• Unit/integration tests cover: precedence resolution, missing required values by mode, invalid value rejection, successful startup with valid mixed inputs.

Sub-tasks & Scope of work

• Define Contract: Define a shared configuration contract for runtime options (network bind/port, hub URL/mode selector, log level, config file path override).
• Implement Loader: Implement source loading from defaults, environment variables (e.g., VOLTA_*), and CLI flags with exact precedence (CLI > env > default).
• Validation: Add normalization and validation (required fields in centralized mode, valid port ranges, valid URL format, enum constraints like log levels).
• Diagnostics: Add startup summary logging (safe-by-default; never print secrets in plain text).
• Documentation: Update command-line parsing, --help text, and README with examples for env-only, CLI-only, and mixed configuration.
• Testing: Add test suite for config resolution, precedence, and validation errors.