Add story 42: Deterministic worktree management via REST/MCP API

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/stories/upcoming/42_deterministic_worktree_management_via_rest_mcp_api.md

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+---
+name: Deterministic Worktree Management via REST/MCP API
+test_plan: pending
+---
+
+# Story 42: Deterministic Worktree Management via REST/MCP API
+
+## User Story
+
+As a developer running multiple agents, I want worktree creation and management handled by the Rust binary through the REST/MCP API, so that worktree locations are deterministic and predictable rather than at the discretion of LLM agents.
+
+## Acceptance Criteria
+
+- [ ] Worktrees are created under `.story_kit/worktrees/` inside the project directory
+- [ ] `.story_kit/worktrees/` contents are gitignored
+- [ ] Worktree creation is exposed through the REST/MCP API with deterministic naming (e.g. based on story ID)
+- [ ] Agents no longer create worktrees themselves — they call the API and receive the worktree path
+- [ ] Existing `start_agent` flow uses the new worktree management instead of ad-hoc creation
+- [ ] Worktree listing is available via API so users can see what worktrees exist
+- [ ] Worktree cleanup/removal is available via API
+
+## Out of Scope
+
+- TBD