feat(P06): 3-tier versioning, branch hierarchy enforcement, ARCHITECTURE-PLAN synthesis

---ci---
phase: 6
milestone: v0.5
status: complete
decisions:
  - id: D-006
    decision: Research as intermediate work product
    rationale: Conclusions update .ci/ files; full research doc intentionally not preserved
    confidence: 0.90
  - id: D-007
    decision: Branch hierarchy enforcement: main > milestone > phase
    rationale: Prevents out-of-order merges and semantically wrong tags
    confidence: 0.92
  - id: D-008
    decision: 3-tier versioning: NFR/feature/schema-breaking
    rationale: Patch per phase (NFR/feature) or minor per phase (schema-breaking); milestone gets minor (feature) or major (schema-breaking)
    confidence: 0.95
requirements:
  covered: [VER-06, BRANCH-01, BRANCH-02, ARCH-01]
---/ci---

- Synthesize ARCHITECTURE-PLAN.md into .ci/ci/ARCHITECTURE.md (expanded 51→230 lines)
- Add D-006/D-007/D-008 to .ci/ci/PROJECT.md key decisions table
- Delete ARCHITECTURE-PLAN.md after synthesis
- Rewrite ship.md with 3-tier versioning model + branch hierarchy merge flows
- Rewrite branch-strategy.md with 3-tier versioning + branch hierarchy + version validation
- Add MilestoneType to config types
- Replace isNfrMilestone() with getMilestoneType() returning nfr|feature|schema-breaking
- Add validateMergeOrder(), mergeMilestoneBranch(), computeMilestoneTag() to GitBranch
- Add computeShipVersion(), validateVersionOrder(), resolveMergeTarget() to ship command
- Remove hardcoded v0.5. from error-recovery rollback
- Create .githooks/pre-push for semver ordering + branch hierarchy validation
- Add 15 new tests (370 total, all passing)

src/core/git-branch.test.ts

@@ -137,4 +137,86 @@ describe("GitBranch", () => {
       expect(result.name).toMatch(/^phase\/01-/);
     });
   });
+
+  describe("validateMergeOrder", () => {
+    it("rejects phase → main when milestone branch exists", () => {
+      const gitBranch = new GitBranch(repoDir);
+      gitBranch.createMilestoneBranch("v0.5", "baseline");
+      gitBranch.createPhaseBranch(1, "auth");
+      execSync(`git checkout main`, { cwd: repoDir, stdio: "pipe" });
+
+      const result = gitBranch.validateMergeOrder("phase/01-auth", "main");
+      expect(result.valid).toBe(false);
+      expect(result.reason).toContain("milestone");
+    });
+
+    it("allows phase → milestone branch", () => {
+      const gitBranch = new GitBranch(repoDir);
+      gitBranch.createMilestoneBranch("v0.5", "baseline");
+      gitBranch.createPhaseBranch(1, "auth");
+      execSync(`git checkout milestone/v0.5-baseline`, { cwd: repoDir, stdio: "pipe" });
+
+      const result = gitBranch.validateMergeOrder("phase/01-auth", "milestone/v0.5-baseline");
+      expect(result.valid).toBe(true);
+    });
+
+    it("allows phase → main when no milestone branch exists", () => {
+      const gitBranch = new GitBranch(repoDir);
+      gitBranch.createPhaseBranch(1, "auth");
+      execSync(`git checkout main`, { cwd: repoDir, stdio: "pipe" });
+
+      const result = gitBranch.validateMergeOrder("phase/01-auth", "main");
+      expect(result.valid).toBe(true);
+    });
+
+    it("allows hotfix → main", () => {
+      const gitBranch = new GitBranch(repoDir);
+      gitBranch.createMilestoneBranch("v0.5", "baseline");
+      execSync(`git checkout -b hotfix/critical-fix main`, { cwd: repoDir, stdio: "pipe" });
+      fs.writeFileSync(path.join(repoDir, "fix.txt"), "fix");
+      execSync(`git add . && git commit -m "hotfix: critical fix"`, { cwd: repoDir, stdio: "pipe" });
+      execSync(`git checkout main`, { cwd: repoDir, stdio: "pipe" });
+
+      const result = gitBranch.validateMergeOrder("hotfix/critical-fix", "main");
+      expect(result.valid).toBe(true);
+    });
+  });
+
+  describe("computeMilestoneTag", () => {
+    it("computes next minor for feature milestone", () => {
+      execSync(`git tag -a v0.5.1 -m "v0.5.1"`, { cwd: repoDir, stdio: "pipe" });
+      execSync(`git tag -a v0.5.2 -m "v0.5.2"`, { cwd: repoDir, stdio: "pipe" });
+
+      const gitBranch = new GitBranch(repoDir);
+      const tag = gitBranch.computeMilestoneTag("feature");
+      expect(tag).toBe("v0.6.0");
+    });
+
+    it("computes next major for schema-breaking 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");
+      expect(tag).toBe("v1.0.0");
+    });
+
+    it("starts from v0.1.0 when no tags exist", () => {
+      const gitBranch = new GitBranch(repoDir);
+      const tag = gitBranch.computeMilestoneTag("feature");
+      expect(tag).toBe("v0.1.0");
+    });
+  });
+
+  describe("mergeMilestoneBranch", () => {
+    it("rejects merge when unmerged phase branches remain", () => {
+      const gitBranch = new GitBranch(repoDir);
+      gitBranch.createMilestoneBranch("v0.5", "baseline");
+      gitBranch.createPhaseBranch(1, "auth");
+      execSync(`git checkout main`, { cwd: repoDir, stdio: "pipe" });
+
+      const result = gitBranch.mergeMilestoneBranch("milestone/v0.5-baseline", "main", true);
+      expect(result.success).toBe(false);
+      expect(result.message).toContain("unmerged");
+    });
+  });
 });