M5: action gate + review inbox — edit distance captured for real

SharedKernel:
- ActionRisk (risk lives on the action) + GatePolicy (the pure autonomy x risk matrix:
  Read never holds, Draft/Publish hold unless Autonomous, Destructive ALWAYS holds).
- IActionGate (AgentActionProposal -> execute|hold) and IBoardWriter.AttachArtifactAsync.

Governance:
- ReviewItem (held action: artifact, child titles, trace, decision, edit distance) in a new
  review_items table (AddReviewItems migration).
- ActionGate: hold -> ReviewItem + "action.held" audit; autonomous -> execute + audit.
- HeldActionExecutor: writes the artifact onto the task and creates the child tasks via
  IBoardWriter (implemented by OrgBoard — no cross-module table access).
- Review inbox API: GET /api/governance/reviews (scope-filtered to where the caller may
  approve), POST /reviews/{id}/approve (optional edited content/children -> normalized
  edit distance recorded — the north-star metric), POST /reviews/{id}/sendback. Deciding
  twice is 409; Members are 403.

Assembler:
- OutputParser (numbered-list child titles, conservative) and the executor now hands every
  completed run's proposal to the gate.

OrgBoard: WorkItem.AttachArtifact + BoardWriter.AttachArtifactAsync.

Verified: build green; ArchitectureTests 8/8; IntegrationTests 41/41 incl. the full M5
acceptance — Aria (gated) proposes a spec, it waits in the inbox with its trace, a Member is
403'd, the owner edits-and-approves, the spec + four child stories land on the board, edit
distance > 0 is recorded and audited; Quill (autonomous) executes straight to the board;
destructive holds even for an autonomous seat and can be sent back. Plus the GatePolicy matrix.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/TeamUp.IntegrationTests/GatePolicyTests.cs

@@ -0,0 +1,26 @@
+using TeamUp.SharedKernel.Access;
+using Xunit;
+
+namespace TeamUp.IntegrationTests;
+
+/// <summary>The gate decision matrix — pure policy, no database.</summary>
+public sealed class GatePolicyTests
+{
+    [Theory]
+    // Read never holds.
+    [InlineData(Autonomy.DraftOnly, ActionRisk.Read, false)]
+    [InlineData(Autonomy.Gated, ActionRisk.Read, false)]
+    [InlineData(Autonomy.Autonomous, ActionRisk.Read, false)]
+    // Draft/Publish hold unless the seat is Autonomous.
+    [InlineData(Autonomy.DraftOnly, ActionRisk.Draft, true)]
+    [InlineData(Autonomy.Gated, ActionRisk.Draft, true)]
+    [InlineData(Autonomy.Autonomous, ActionRisk.Draft, false)]
+    [InlineData(Autonomy.Gated, ActionRisk.Publish, true)]
+    [InlineData(Autonomy.Autonomous, ActionRisk.Publish, false)]
+    // Destructive ALWAYS holds — whatever the autonomy. The backstop.
+    [InlineData(Autonomy.DraftOnly, ActionRisk.Destructive, true)]
+    [InlineData(Autonomy.Gated, ActionRisk.Destructive, true)]
+    [InlineData(Autonomy.Autonomous, ActionRisk.Destructive, true)]
+    public void Gate_matrix(Autonomy autonomy, ActionRisk risk, bool shouldHold) =>
+        Assert.Equal(shouldHold, GatePolicy.ShouldHold(autonomy, risk));
+}