feat: add driving flag to relationship query responses

Add computed driving flag to all relationship queries (list_relationships,
get_predecessors, get_successors). A relationship is marked as driving when
the predecessor's early end date plus lag determines the successor's early
start date.

Changes:
- Add early_start_date and early_end_date columns to activities schema
- Parse early dates from TASK table in XER files
- Implement is_driving_relationship() helper with 24hr tolerance for
  calendar gaps
- Update all relationship queries to compute and return driving flag
- Add contract and unit tests for driving flag functionality
- Update spec, contracts, and documentation

specs/001-schedule-tools/spec.md

@@ -55,10 +55,10 @@ As an AI assistant user, I want to query the relationships (dependencies) betwee
 
 **Acceptance Scenarios**:
 
-1. **Given** an XER file is loaded, **When** I request predecessors for an activity, **Then** I receive a list of predecessor activities with their relationship types (FS, SS, FF, SF) and lag values
-2. **Given** an XER file is loaded, **When** I request successors for an activity, **Then** I receive a list of successor activities with their relationship types and lag values
+1. **Given** an XER file is loaded, **When** I request predecessors for an activity, **Then** I receive a list of predecessor activities with their relationship types (FS, SS, FF, SF), lag values, and driving flag
+2. **Given** an XER file is loaded, **When** I request successors for an activity, **Then** I receive a list of successor activities with their relationship types, lag values, and driving flag
 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
+4. **Given** an XER file is loaded, **When** I request all relationships, **Then** I receive the dependency network with relationship types, lag values, and driving flags (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
 
@@ -97,7 +97,7 @@ As an AI assistant user, I want to get a high-level summary of the project sched
 - **FR-002**: System MUST expose an MCP tool to load an XER file from a specified file path
 - **FR-003**: System MUST expose an MCP tool to list all activities with filtering options (by date range, by WBS, by activity type)
 - **FR-004**: System MUST expose an MCP tool to retrieve detailed information for a specific activity by ID
-- **FR-005**: System MUST expose an MCP tool to query predecessor and successor relationships for any activity
+- **FR-005**: System MUST expose an MCP tool to query predecessor and successor relationships for any activity, including relationship type, lag value, and driving flag
 - **FR-006**: System MUST expose an MCP tool to retrieve project summary information (name, data date, plan dates, activity count)
 - **FR-007**: System MUST expose an MCP tool to list milestone activities
 - **FR-008**: System MUST expose an MCP tool to identify the critical path
@@ -140,6 +140,7 @@ As an AI assistant user, I want to get a high-level summary of the project sched
 - 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
 - Q: Should project summary include the data date? → A: Yes, include the data date (schedule "as-of" date) in project summary response
+- Q: Should relationship queries return the driving flag? → A: Yes, include the driving property in all relationship responses (predecessors, successors, list relationships)
 
 ## Assumptions