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

CLAUDE.md

@@ -3,6 +3,7 @@
 Auto-generated from all feature plans. Last updated: 2026-01-06
 
 ## Active Technologies
+- SQLite in-memory database (001-schedule-tools)
 
 - Python 3.14 + mcp (MCP SDK), sqlite3 (stdlib) (001-schedule-tools)
 
@@ -22,6 +23,7 @@ cd src [ONLY COMMANDS FOR ACTIVE TECHNOLOGIES][ONLY COMMANDS FOR ACTIVE TECHNOLO
 Python 3.14: Follow standard conventions
 
 ## Recent Changes
+- 001-schedule-tools: Added Python 3.14 + mcp (MCP SDK), sqlite3 (stdlib)
 
 - 001-schedule-tools: Added Python 3.14 + mcp (MCP SDK), sqlite3 (stdlib)