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/data-model.md

@@ -56,6 +56,8 @@ A unit of work in the schedule.
 | task_type | enum | Yes | TT_Task, TT_Mile, TT_LOE, TT_WBS, TT_Rsrc |
 | target_start_date | datetime | No | Planned start |
 | target_end_date | datetime | No | Planned finish |
+| early_start_date | datetime | No | Calculated early start (for driving computation) |
+| early_end_date | datetime | No | Calculated early finish (for driving computation) |
 | act_start_date | datetime | No | Actual start |
 | act_end_date | datetime | No | Actual finish |
 | total_float_hr_cnt | float | No | Total float in hours |
@@ -88,9 +90,23 @@ A dependency link between two activities.
 | pred_task_id | string | Yes | Predecessor activity (the one constraining) |
 | pred_type | enum | Yes | PR_FS, PR_SS, PR_FF, PR_SF |
 | lag_hr_cnt | float | No | Lag time in hours (can be negative) |
+| driving | boolean | No | **Computed** - True if this relationship determines successor's early dates |
 
 **XER Source**: `TASKPRED` table
 
+**Driving Flag Computation** (not stored, computed at query time):
+
+The `driving` flag indicates whether this predecessor relationship constrains the successor activity's early start date. It is computed by comparing dates:
+
+```python
+# For FS (Finish-to-Start) relationships:
+driving = (predecessor.early_end_date + lag_hr_cnt ≈ successor.early_start_date)
+
+# With 1-hour tolerance for floating point arithmetic
+```
+
+This requires JOIN on activity dates when querying relationships.
+
 **Relationship Types**:
 | Code | Name | Meaning |
 |------|------|---------|
@@ -172,6 +188,8 @@ CREATE TABLE activities (
     task_type TEXT NOT NULL,
     target_start_date TEXT,
     target_end_date TEXT,
+    early_start_date TEXT,       -- Used for driving relationship computation
+    early_end_date TEXT,         -- Used for driving relationship computation
     act_start_date TEXT,
     act_end_date TEXT,
     total_float_hr_cnt REAL,