[codex] add workflow aliases and root steps

[thisrohangupta]

Summary

• add workflow and agent as root aliases alongside existing pipeline
• keep workflow id and name active because scoped workflows need first-class identity
• add root-level steps and Go normalization into an implicit default stage
• add samples and README notes for aliases and root steps
• regenerate dist/schema.json and update checked-in Go structs/tests

Why

Jyoti clarified that pipeline = workflow = agent: they are interchangeable names for the same underlying schema and concept. She also called out the need for a fourth reusable level (global, marketplace, or system) that can host workflows/agents/pipelines available across customer accounts.

Brad clarified that placement/scope should be platform resource metadata, not YAML schema metadata. This PR therefore does not add workflow scope, org, or project fields.

Scoped workflows still need stable identity on the workflow object itself. This PR removes the deprecated markers from workflow id and name instead of moving identity exclusively into a resource envelope.

Follow-up

• Stacked connector-input branch: workflow-connector-input
• Stacked draft PR: [codex] add connector input type thisrohangupta/spec#4

Validation

• npm run schema
• npm run golang after creating dist/go for generator output validation
• go test ./yaml -run 'TestParseWorkflowAliases|TestParseWorkflowAliasConflict|TestNormalizedWorkflowRootSteps'
• git diff --check

Note: go test ./yaml currently fails on clean upstream master due to an existing golden mismatch around environment.sequential; the same failure reproduces without this branch.