docs: Create README.md and CHANGELOG.md for initial metrics package release

[jflowers]

What Changed

The opsx/universal-coupling-model branch introduces the foundational metrics package — the first implementation code in the vibe-check repository. This includes:

• 14 production source files defining the universal coupling metrics model
• Public API types: Module, ModuleResult, ModuleGraph, Adapter interface, Registry, ExternalAdapter
• Named metric types: Instability, Abstractness, Distance, LCOM with documented value ranges
• Computation functions: ComputeInstability, ComputeAbstractness, ComputeDistance, ComputeZone
• JSON-RPC 2.0 protocol for external language adapters
• JSON Schema (modulegraph.schema.json) for interchange format validation
• Security: path validation, environment sanitization for subprocess execution

Documentation Needed

README.md (new file)

• Project description and mission (Martin metrics suite for Go)
• Installation instructions (go get github.com/zero-dot-force/vibe-check)
• Quick usage example showing metric computation
• Link to GoDoc
• Link to RFC (unbound-force/discussions/483)
• Architecture overview (two-layer model)
• License (Apache 2.0)

CHANGELOG.md (new file)

• Initial entry for the metrics package addition
• Document all new types, interfaces, and functions
• Note the JSON schema version (1.0)

AGENTS.md cleanup

• Remove placeholder comment on line 237 (<!-- Placeholder — update when go.mod and Makefile are created -->)

Why It Matters

This is the project's first public-facing code. Without a README, developers discovering the repository on GitHub have no landing page. Without a CHANGELOG, the project's versioning story (required by AGENTS.md) has no tracking artifact.

References

• Branch: opsx/universal-coupling-model
• AGENTS.md behavioral rules: lines 185-193 (documentation gate)

Labels: This issue should be labeled docs once the label is created.

[jflowers]

Completed: README.md and CHANGELOG.md were created and shipped in PR #21 (v0.1.0). This is a repo-artifact issue, not a website concern — the public-site docs-sync is tracked separately at unbound-force/website#275.