docs: add implementation plan and clarify no-file-loaded errors

Plan artifacts:
- plan.md: Technical context, constitution check, project structure
- research.md: XER format, MCP SDK, SQLite schema decisions
- data-model.md: Entity definitions and database schema
- contracts/mcp-tools.json: MCP tool schemas (9 tools)
- quickstart.md: Usage guide with examples
- CLAUDE.md: Agent context file

Spec updates:
- Add FR-015: NO_FILE_LOADED error requirement
- Add acceptance scenarios for no-file-loaded errors to US3, US4

CLAUDE.md

@@ -0,0 +1,29 @@
+# xer-mcp Development Guidelines
+
+Auto-generated from all feature plans. Last updated: 2026-01-06
+
+## Active Technologies
+
+- Python 3.14 + mcp (MCP SDK), sqlite3 (stdlib) (001-schedule-tools)
+
+## Project Structure
+
+```text
+src/
+tests/
+```
+
+## Commands
+
+cd src [ONLY COMMANDS FOR ACTIVE TECHNOLOGIES][ONLY COMMANDS FOR ACTIVE TECHNOLOGIES] pytest [ONLY COMMANDS FOR ACTIVE TECHNOLOGIES][ONLY COMMANDS FOR ACTIVE TECHNOLOGIES] ruff check .
+
+## Code Style
+
+Python 3.14: Follow standard conventions
+
+## Recent Changes
+
+- 001-schedule-tools: Added Python 3.14 + mcp (MCP SDK), sqlite3 (stdlib)
+
+<!-- MANUAL ADDITIONS START -->
+<!-- MANUAL ADDITIONS END -->