Add optional version field to global agents.config.yaml

[Copilot]

Allow tracking changes to the global configuration file by adding a version identifier.

Changes

• Schema (config/schema.py): Added optional version: str | None = None to GlobalConfig. A mode="before" validator coerces numeric YAML literals (1, 1.0) to strings and rejects booleans. extra="forbid" is preserved.
• Tests (test_config_loader.py): Updated the empty-dump assertion; added cases for explicit string and numeric-coerced versions.
• Docs (docs/front-matter-spec.md): Listed version in the global optional-properties reference and described its purpose.
• Samples: Added version: "1.0" to the three sample agents.config.yaml files.

Example

# agents.config.yaml
version: "1.0"
model: gpt-4o
timeout: 900

The field is fully optional — existing configs without version continue to load unchanged. No semantics are attached to the value; it is informational metadata for users to track config evolution.