story-kit: create 347_story_mcp_tool_for_shell_command_execution

.story_kit/work/1_backlog/347_story_mcp_tool_for_shell_command_execution.md

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+---
+name: "MCP tool for shell command execution"
+---
+
+# Story 347: MCP tool for shell command execution
+
+## User Story
+
+As a non-Claude agent connected via MCP, I want a shell command tool so that I can run cargo build, npm test, and other commands in my worktree.
+
+## Acceptance Criteria
+
+- [ ] run_command tool — executes a bash command and returns stdout/stderr/exit_code
+- [ ] Command runs in the agent's worktree directory
+- [ ] Supports timeout parameter (default 120s, max 600s)
+- [ ] Sandboxed to worktree — cannot cd outside or access host paths
+- [ ] Returns streaming output for long-running commands
+- [ ] Dangerous commands blocked (rm -rf /, etc.)
+
+## Out of Scope
+
+- TBD