Formalize incremental demand evolution

[JonatasFreireDev]

Summary

This PR formalizes incremental demand evolution while preserving the existing Domain -> Goal -> Feature -> Use Case hierarchy.

What changed

• Added an agent-led demand classification flow for extensions, new Use Cases, new Features, new Goals/Domains, and non-delivery items.
• Extended Feature and Use Case context.md templates with optional relations, traceability, and evolution metadata.
• Updated Artifact Importer mappings to preserve target type, relation, reuse, extension, and impact proposals.
• Updated Evolution, Feature, Use Case, Specification, and orchestrator skills to read existing context and sibling artifacts before creating a delta.
• Added validator and registry support for evolution metadata and import relation validation.
• Added tests for demand mapping persistence, invalid relation values, optional legacy contexts, and registry preservation.
• Updated framework method, bootstrap, starter guidance, adoption docs, and starting-point references.
• Retained and included the previous agent-first code-root discovery changes in this branch.

Compatibility

• Existing contexts without evolution metadata remain valid.
• No product approvals, product decisions, or examples/events content were modified.
• No new CLI command was required; existing import and upgrade boundaries remain authoritative.

Validation

• go test ./...
• go vet ./...
• go run ./cmd/spec-framework validate --product-root examples/events --framework-root .
• git diff --check

Validation result: ready, 0 errors, 0 warnings, 0 notes.