Configuration management

[gloskull]

Motivation

• Introduce a system-wide, schema-first configuration subsystem to validate operational changes before activation and enable deterministic hot-reload flows.
• Gate runtime changes to services with explicit hot_reload flags to avoid unsafe in-place mutations during live rollouts.
• Provide compact change events for monitoring and to drive blue-green / canary automation and runbooks.

Description

• Add src/config.rs implementing SystemConfig, ServiceConfig, MonitoringConfig, DeploymentConfig, a ConfigManager with new, active, history, and reload methods, and pure validators validate_config and validate_reload that enforce version monotonicity, schema bounds, and hot-reload gating.
• Export the new module from the crate root by adding pub mod config; to src/lib.rs.
• Add focused unit tests in tests/config_management_test.rs that cover successful reloads, schema validation failures, non-monotonic version rejection, disabled-hot-reload gating, duplicate service detection, and invalid canary settings.
• Add documentation and an operational runbook at docs/configuration-management.md and docs/runbooks/configuration-hot-reload.md describing the hot-reload flow, operational bounds, rollout, and rollback guidance.

Testing

• Ran cargo test --test config_management_test, which executed 4 tests and all tests passed.
• Ran cargo check, which completed successfully (no errors reported).

Closes #77