fix(start-work): honor explicit plan name and strip ultrawork keywords

When user types '/start-work my-plan ultrawork', the hook now:

1. Extracts plan name from <user-request> section
2. Strips ultrawork/ulw keywords from the plan name
3. Searches for matching plan (exact then partial match)
4. Uses the matched plan instead of resuming stale boulder state

This fixes the bug where '/start-work [PLAN] ultrawork' would:
- Include 'ultrawork' as part of the plan name argument
- Ignore the explicit plan and resume an old stale plan from boulder.json

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

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

@@ -236,5 +236,148 @@ describe("start-work hook", () => {
       expect(output.parts[0].text).toContain("Ask the user")
       expect(output.parts[0].text).not.toContain("Which plan would you like to work on?")
     })
+
+    test("should select explicitly specified plan name from user-request, ignoring existing boulder state", async () => {
+      // #given - existing boulder state pointing to old plan
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      // Old plan (in boulder state)
+      const oldPlanPath = join(plansDir, "old-plan.md")
+      writeFileSync(oldPlanPath, "# Old Plan\n- [ ] Old Task 1")
+
+      // New plan (user wants this one)
+      const newPlanPath = join(plansDir, "new-plan.md")
+      writeFileSync(newPlanPath, "# New Plan\n- [ ] New Task 1")
+
+      // Set up stale boulder state pointing to old plan
+      const staleState: BoulderState = {
+        active_plan: oldPlanPath,
+        started_at: "2026-01-01T10:00:00Z",
+        session_ids: ["old-session"],
+        plan_name: "old-plan",
+      }
+      writeBoulderState(TEST_DIR, staleState)
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [
+          {
+            type: "text",
+            text: `Start Sisyphus work session
+<user-request>
+new-plan
+</user-request>`,
+          },
+        ],
+      }
+
+      // #when - user explicitly specifies new-plan
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should select new-plan, NOT resume old-plan
+      expect(output.parts[0].text).toContain("new-plan")
+      expect(output.parts[0].text).not.toContain("RESUMING")
+      expect(output.parts[0].text).not.toContain("old-plan")
+    })
+
+    test("should strip ultrawork/ulw keywords from plan name argument", async () => {
+      // #given - plan with ultrawork keyword in user-request
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      const planPath = join(plansDir, "my-feature-plan.md")
+      writeFileSync(planPath, "# My Feature Plan\n- [ ] Task 1")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [
+          {
+            type: "text",
+            text: `Start Sisyphus work session
+<user-request>
+my-feature-plan ultrawork
+</user-request>`,
+          },
+        ],
+      }
+
+      // #when - user specifies plan with ultrawork keyword
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should find plan without ultrawork suffix
+      expect(output.parts[0].text).toContain("my-feature-plan")
+      expect(output.parts[0].text).toContain("Auto-Selected Plan")
+    })
+
+    test("should strip ulw keyword from plan name argument", async () => {
+      // #given - plan with ulw keyword in user-request
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      const planPath = join(plansDir, "api-refactor.md")
+      writeFileSync(planPath, "# API Refactor\n- [ ] Task 1")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [
+          {
+            type: "text",
+            text: `Start Sisyphus work session
+<user-request>
+api-refactor ulw
+</user-request>`,
+          },
+        ],
+      }
+
+      // #when
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should find plan without ulw suffix
+      expect(output.parts[0].text).toContain("api-refactor")
+      expect(output.parts[0].text).toContain("Auto-Selected Plan")
+    })
+
+    test("should match plan by partial name", async () => {
+      // #given - user specifies partial plan name
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      const planPath = join(plansDir, "2026-01-15-feature-implementation.md")
+      writeFileSync(planPath, "# Feature Implementation\n- [ ] Task 1")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [
+          {
+            type: "text",
+            text: `Start Sisyphus work session
+<user-request>
+feature-implementation
+</user-request>`,
+          },
+        ],
+      }
+
+      // #when
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should find plan by partial match
+      expect(output.parts[0].text).toContain("2026-01-15-feature-implementation")
+      expect(output.parts[0].text).toContain("Auto-Selected Plan")
+    })
   })
 })