feat: add user-defined workflows

[castrojo]

Summary

• add workflow CLI commands for user-defined markdown/YAML workflows
• load project, user, and built-in workflow definitions with strict YAML field validation
• support backward transitions, conditional skips, multi-sink terminal states, persistence, status, and resume/restart behavior
• ship and document the built-in marketing-ideation non-code workflow example

Fixes #58

Verification

• go test -count=1 ./internal/workflow ./internal/customworkflow ./cmd
• PATH=/home/hive/workspace/hivecommons/bin:/home/hive/workspace/hivecommons/go/bin:$PATH go test ./...
• smoke: built CLI, ran workflow new, backward workflow goto, and workflow status for marketing-ideation

— hive: backend=omp

🐝 Hive Agent: contributor | SHA: 7446ac7

[kubestellar-prow]

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[kubestellar-prow]

PR needs rebase.

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