fix(start-work): remove worktree setup prompt when unspecified, add strong worktree active instructions

When no worktree is specified in boulder, stop injecting 'Worktree Setup Required'
instructions. When worktree IS present, inject emphatic instructions ensuring the
agent and all subagents operate exclusively within the worktree directory.

src/hooks/start-work/index.test.ts

@@ -417,7 +417,7 @@ describe("start-work hook", () => {
       detectSpy.mockRestore()
     })
 
-    test("should inject model-decides instructions when no --worktree flag", async () => {
+    test("should NOT inject worktree instructions when no --worktree flag", async () => {
       // given - single plan, no worktree flag
       const plansDir = join(testDir, ".sisyphus", "plans")
       mkdirSync(plansDir, { recursive: true })
@@ -431,10 +431,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-123" }, output)
 
-      // then - model-decides instructions should appear
-      expect(output.parts[0].text).toContain("Worktree Setup Required")
-      expect(output.parts[0].text).toContain("git worktree list --porcelain")
-      expect(output.parts[0].text).toContain("git worktree add")
+      // then - no worktree instructions should appear
+      expect(output.parts[0].text).not.toContain("Worktree Setup Required")
+      expect(output.parts[0].text).not.toContain("Worktree Active")
+      expect(output.parts[0].text).not.toContain("git worktree list --porcelain")
     })
 
     test("should inject worktree path when --worktree flag is valid", async () => {
@@ -452,8 +452,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-123" }, output)
 
-      // then - validated path shown, no model-decides instructions
-      expect(output.parts[0].text).toContain("**Worktree**: /validated/worktree")
+      // then - strong worktree active instructions shown
+      expect(output.parts[0].text).toContain("Worktree Active")
+      expect(output.parts[0].text).toContain("/validated/worktree")
+      expect(output.parts[0].text).toContain("subagent")
       expect(output.parts[0].text).not.toContain("Worktree Setup Required")
     })
 
@@ -548,8 +550,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-789" }, output)
 
-      // then - shows existing worktree, no model-decides instructions
+      // then - shows strong worktree active instructions
+      expect(output.parts[0].text).toContain("Worktree Active")
       expect(output.parts[0].text).toContain("/existing/wt")
+      expect(output.parts[0].text).toContain("subagent")
       expect(output.parts[0].text).not.toContain("Worktree Setup Required")
     })
   })