feat(FN-1430): add worktree execution boundaries to prevent out-of-scope modifications

- Add worktree-aware path boundaries in agent factory to prevent cross-task contamination
- Sync core agent prompts with worktree boundary guidance for consistent enforcement
- Add boundary guidance to executor prompts so agents understand their scope
- Fix TypeScript types for worktree boundary wrapping
- Add comprehensive tests for boundary wrapping behavior
- Create changeset for @gsxdsm/fusion (minor)

packages/engine/src/executor.ts

@@ -180,6 +180,17 @@ Documents are versioned — each write creates a new revision. Use meaningful ke
 - Use conventional commit messages prefixed with the task ID
 - Do NOT commit broken or half-implemented code
 
+## Worktree Boundaries
+
+You are running in an **isolated git worktree**. This means:
+
+- **All code changes must be made inside the current worktree directory.** Do not modify files outside the worktree — the worktree is your isolated execution environment.
+- **Exception — Project memory:** You MAY read and write to .fusion/memory.md at the project root to save durable project learnings (architecture patterns, conventions, pitfalls).
+- **Exception — Task attachments:** You MAY read files under .fusion/tasks/{taskId}/attachments/ at the project root for context screenshots and documents attached to this task.
+- **Shell commands** run inside the worktree by default. Avoid using cd to navigate outside the worktree.
+
+If you attempt to write to a path outside the worktree, the file tools will reject the operation with an error explaining the boundary.
+
 ## Guardrails
 - Treat the File Scope in PROMPT.md as the expected starting scope, not a hard boundary when quality gates fail
 - Read "Context to Read First" files before starting
@@ -3486,6 +3497,15 @@ ${reviewLevel >= 2 ? `After implementing + committing each step, call:
 \`review_step(step=N, type="code", step_name="...", baseline="<SHA from before step>")\`` : ""}
 ${reviewLevel >= 3 ? `After tests, also call review_step with type="code" for test review.` : ""}
 
+## Worktree Boundaries
+
+You are running in an **isolated git worktree**. This means:
+
+- **All code changes must be made inside the current worktree directory.** Do not modify files outside the worktree.
+- **Exception — Project memory:** You MAY read and write to \`.fusion/memory.md\` at the project root to save durable project learnings.
+- **Exception — Task attachments:** You MAY read files under \`.fusion/tasks/{taskId}/attachments/\` at the project root for context.
+- **Shell commands** run inside the worktree by default. Avoid using \`cd\` to navigate outside the worktree.
+
 ## Begin
 
 ${hasProgress