docs(plugins): add execution spine contract, health schema, handoff packet, retry/dead-letter policy

docs/plugins/EXECUTION_SPINE_CONTRACT.md

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+# Plugin Execution Spine Contract (v1)
+
+## Flow
+`propose -> approve(if risky) -> execute -> verify`
+
+## ACK-first
+Actionable requests SHOULD emit immediate ACK before long-running work.
+
+## Goals
+- Lean core, stronger plugins
+- Predictable safety behavior
+- Auditable execution trail

docs/plugins/HANDOFF_PACKET_SCHEMA_V1.md

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+# Resumable Handoff Packet (v1)
+
+## Purpose
+Allow model/agent/plugin swap mid-run without continuity loss.
+
+## Core fields
+- `runId`
+- `goalId`
+- `currentStep`
+- `acceptance`
+- `artifacts[]`
+- `status`
+- `updatedAt`

docs/plugins/PLUGIN_HEALTH_SCHEMA.md

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+# Plugin Health Schema (v1)
+
+## Status enum
+- `healthy`
+- `degraded`
+- `blocked`
+
+## Required fields
+- `status`
+- `lastError` (nullable)
+- `nextAction` (nullable)
+- `updatedAt`

docs/plugins/RETRY_DEADLETTER_POLICY.md

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+# Retry + Dead-letter Policy (v1)
+
+## Retry
+- Bounded retries only (no infinite loops)
+- Explicit retry count + backoff
+
+## Dead-letter
+- Failed items after max retries move to dead-letter queue
+- Must include failure reason + next operator action