Add Metadoor workflow story and zigttp:workflow design spec

[srdjan]

Motivation

• Lift the Metadoor HAL-typed long-running workflow model into a first-class zigttp virtual module so workflows can be defined, versioned, and executed alongside handlers using existing durable primitives.
• Capture requirements for deterministic replay, typed step contracts, durable timers/signals, idempotent activity dispatch, and safe version migration in a single design artifact.

Description

• Add a new design document docs/metadoor-workflow-spec.md that defines the product story and a draft API for zigttp:workflow.
• The spec includes an API sketch (defineWorkflow, startWorkflow, signalWorkflow, queryWorkflow, cancelWorkflow, continueAsNew), the HAL-typed validation rules, durable execution mapping (event log, checkpoints, timers, signal inbox, activity queue), and determinism contracts.
• The document defines the workflow state machine, activity model (pure, durable, human tasks), versioning and migration strategies, observability and security considerations, failure semantics, acceptance criteria, a phased implementation plan, and open design questions.

Testing

• Created and committed the file via git add docs/metadoor-workflow-spec.md && git commit -m 'add metadoor workflow story and zigttp module spec', and verified contents with nl -ba docs/metadoor-workflow-spec.md | sed -n '1,260p'; these commands completed successfully.
• No automated runtime or unit tests were run because this is a documentation-only change.

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Codex Task