refactor(P06): rename milestone type schema-breaking → major, reinforce release flow

---ci---
phase: 6
milestone: v0.10
status: execute
decisions:
  - id: D-001
    decision: Rename MilestoneType schema-breaking to major for clarity
    rationale: Major better describes the semver impact (major version bump) and aligns with standard semver terminology
    confidence: 0.95
    alternatives: [schema-breaking, breaking, major-change]
  - id: D-002
    decision: Add autopilot rules, PR+QA gates, and merge validation to ship workflow
    rationale: Release flow was documented but not enforced in the workflow. Zero-HITL rules, branch hierarchy validation, and coreci packaging steps ensure consistent releases
    confidence: 0.90
    alternatives: [keep-as-documentation-only, add-to-AGENTS.md-only]
---/ci---

src/core/git-branch.test.ts

@@ -192,11 +192,11 @@ describe("GitBranch", () => {
       expect(tag).toBe("v0.6.0");
     });
 
-    it("computes next major for schema-breaking milestone", () => {
+    it("computes next major for major milestone", () => {
       execSync(`git tag -a v0.5.1 -m "v0.5.1"`, { cwd: repoDir, stdio: "pipe" });
 
       const gitBranch = new GitBranch(repoDir);
-      const tag = gitBranch.computeMilestoneTag("schema-breaking");
+      const tag = gitBranch.computeMilestoneTag("major");
       expect(tag).toBe("v1.0.0");
     });