Add AgentPlane bridge spec

Author: mdheller

docs/sourceos/AGENTPLANE_BRIDGE.md

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+# TurtleTerm AgentPlane Bridge
+
+## Purpose
+
+AgentPlane owns evidence-producing execution. TurtleTerm should promote terminal/session work into AgentPlane bundles when execution needs deterministic validation, placement, run, evidence, and replay.
+
+## Bridge flow
+
+```text
+TurtleTerm session or command proposal
+  → create AgentPlane bundle
+  → validate bundle
+  → select executor
+  → run bundle
+  → collect evidence artifacts
+  → link artifacts to TurtleTerm receipt
+```
+
+## AgentPlane artifacts
+
+TurtleTerm should link or embed references to:
+
+- ValidationArtifact
+- PlacementDecision
+- RunArtifact
+- ReplayArtifact
+- PromotionArtifact
+- ReversalArtifact
+- SessionArtifact
+
+## Candidate commands
+
+```bash
+turtle-agentplane create-bundle --session <session-id>
+turtle-agentplane validate <bundle>
+turtle-agentplane run <bundle>
+turtle-agentplane attach-artifacts --session <session-id> --artifacts <dir>
+```
+
+## When to use AgentPlane
+
+- agent execution beyond human-mediated terminal capture
+- high-risk host execution
+- CI/release actions
+- networked execution
+- reproducibility required
+- rollback/replay required
+- policy obligations require evidence
+
+## Non-goals
+
+- replacing AgentPlane runners
+- bypassing AgentPlane validation
+- hiding failed evidence gates