story-kit: start 78_story_create_spike_mcp_tool

.story_kit/work/2_current/78_story_create_spike_mcp_tool.md

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+---
+name: "Create spike MCP tool"
+---
+
+# Story 78: Create spike MCP tool
+
+## User Story
+
+As a user, I want to create spikes via the MCP API, so that spike work items are created consistently with proper YAML front matter just like stories and bugs.
+
+## Acceptance Criteria
+
+- [ ] A create_spike MCP tool exists with name and description parameters
+- [ ] create_spike_file function in server/src/http/workflow.rs generates a properly formatted spike markdown file with YAML front matter
+- [ ] The spike file is created in .story_kit/work/1_upcoming/ with the naming pattern N_spike_slug.md
+- [ ] The name value is quoted in front matter to handle YAML-special characters
+- [ ] cargo clippy and cargo test pass
+
+## Out of Scope
+
+- TBD