Extend Capabilities with System Requirements

[jbouder]

Tasks

• Define and document the requirements block in the canonical capability manifest JSON Schema, including all field types, allowed enum values, and optionality rules
• Implement a cross-platform requirement evaluator that resolves the host OS (normalized to windows / macos / linux), architecture, available memory, free disk, and installed runtime versions
• Integrate requirement evaluation into the Launcher's capability load path
• Emit a clear, actionable error message per failed requirement (e.g. "Requires macOS 13+, current version is 12.6") without crashing the Launcher
• Update the capabilities authoring documentation with the new schema fields, valid values, and at least two worked examples

Acceptance Criteria

• A capability manifest declaring requirements with valid OS, arch, and runtime fields passes schema validation without errors on Windows, macOS, and Linux
• A capability whose requirements are satisfied by the host machine loads and executes normally — no regressions for capabilities without a requirements block
• A capability whose requirements are not satisfied surfaces a human-readable error identifying each unmet constraint before execution is attempted
• A manifest with an unrecognized OS identifier or unsupported field name produces a validation warning but does not prevent the capability from loading on a compatible host