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

specs/001-schedule-tools/spec.md

@@ -60,6 +60,7 @@ As an AI assistant user, I want to query the relationships (dependencies) betwee
 3. **Given** an activity has no predecessors, **When** I request its predecessors, **Then** I receive an empty list (not an error)
 4. **Given** an XER file is loaded, **When** I request all relationships, **Then** I receive the dependency network (limited to 100 by default) with pagination metadata
 5. **Given** an XER file is loaded, **When** I request relationships with offset and limit parameters, **Then** I receive the specified page of results
+6. **Given** no XER file is loaded, **When** I request relationships or predecessors/successors, **Then** I receive a clear error message indicating no file is loaded
 
 ---
 
@@ -76,6 +77,7 @@ As an AI assistant user, I want to get a high-level summary of the project sched
 1. **Given** an XER file is loaded, **When** I request the project summary, **Then** I receive project name, start date, finish date, and total activity count
 2. **Given** an XER file with milestones, **When** I request milestones, **Then** I receive a list of milestone activities with their target dates
 3. **Given** an XER file is loaded, **When** I request the critical path, **Then** I receive the sequence of activities that determine the project end date
+4. **Given** no XER file is loaded, **When** I request project summary, milestones, or critical path, **Then** I receive a clear error message indicating no file is loaded
 
 ---
 
@@ -105,6 +107,7 @@ As an AI assistant user, I want to get a high-level summary of the project sched
 - **FR-012**: System MUST handle XER files with multiple projects by requiring explicit project selection; single-project files auto-select the only project
 - **FR-013**: System MUST implement pagination for list queries with a default limit of 100 items, supporting offset and limit parameters
 - **FR-014**: System MUST return pagination metadata (total_count, has_more, offset, limit) with all paginated responses
+- **FR-015**: System MUST return an informative error (NO_FILE_LOADED) when any query tool is invoked before an XER file has been successfully loaded
 
 ### Key Entities
 
@@ -135,6 +138,7 @@ As an AI assistant user, I want to get a high-level summary of the project sched
 - Q: How should critical path be determined? → A: Use P6's stored critical flags from XER data
 - Q: How should multi-project XER files be handled? → A: Require explicit project selection if multiple exist
 - Q: Should calendar data be exposed as queryable? → A: Internal use only (not exposed as queryable)
+- Q: What happens when any query tool is called without a file loaded? → A: Return informative error indicating no XER file is loaded; applies to all tools except load_xer
 
 ## Assumptions