Add harness vision & roadmap design doc

[MuJiongan]

What

Adds docs/harness-vision.md — a design doc capturing the direction for evolving Ensemble from a workflow builder into a custom agent harness, and links it from the README.

Why

Direction-setting from a brainstorm. The doc is grounded in the current architecture (real file paths, table names, the node REGISTRY, null-propagation, the design invariants) so it stands on its own and stays useful when revisited.

Contents

• The reframe — graph builder → agent-system compiler + runtime + registry, across three layers (authoring / execution semantics / distribution)
• Where we are today — a grounded snapshot of the current system
• Five workstreams, each as current state → gap → concrete moves:
  • Tool creation (synthesized tools, tool-from-workflow, policy wrappers)
  • Compaction (pluggable strategies, ctx.memory, compaction-as-hook)
  • LLM primitives (structured output, sample/judge/vote, classify/embed, spawn, budgets)
  • Hooks & loop control (lifecycle events, gate/mutate/repair, declarative stop conditions)
  • Packaging & reuse (subworkflow node, local library, orchestrator recruits)
• Natural extensions — eval/regression, triggers, typed contracts, replay
• Invariants to preserve — pull-not-push, plan-not-do, acyclic, isolation-≠-sandbox
• A suggested arc and an open-decisions list

Notes

No code changes — documentation only.

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