[blainemotsinger]

Adds a workflows category to the knowledge store so procedural knowledge can be fetched on demand instead of loaded as a plugin skill.

• ValidCategories and CategoryPrefix gain workflows / wf
• Category error messages and tool descriptions are generated from ValidCategories so the lists cannot drift
• README, examples, and the install skill description updated
• Validation test extended; an integration test writes and reads a workflows entry

Verification: gofmt -l clean, go vet ./..., go test ./... all pass.

Closes #37