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/research.md

@@ -198,3 +198,80 @@ class PaginationMetadata:
 | -32004 | PROJECT_SELECTION_REQUIRED | Multi-project file without selection |
 | -32005 | ACTIVITY_NOT_FOUND | Requested activity ID doesn't exist |
 | -32006 | INVALID_PARAMETER | Bad filter/pagination parameters |
+
+## Driving Relationship Flag
+
+**Research Date**: 2026-01-06
+
+### Question: What field in the XER TASKPRED table contains the driving relationship flag?
+
+**Finding**: The TASKPRED table in P6 XER files does NOT contain a direct `driving_flag` field.
+
+**Evidence**: Analysis of sample XER file (S48019R - Proposal Schedule):
+```
+%F  task_pred_id  task_id  pred_task_id  proj_id  pred_proj_id  pred_type  lag_hr_cnt  comments  float_path  aref  arls
+```
+
+Fields available:
+- `task_pred_id` - Unique relationship identifier
+- `task_id` - Successor activity ID
+- `pred_task_id` - Predecessor activity ID
+- `proj_id` / `pred_proj_id` - Project identifiers
+- `pred_type` - Relationship type (PR_FS, PR_SS, PR_FF, PR_SF)
+- `lag_hr_cnt` - Lag duration in hours
+- `comments` - User comments
+- `float_path` - Float path indicator (contains dates, not boolean)
+- `aref` / `arls` - Activity reference dates
+
+### Question: Where is driving/critical path information stored in P6 XER files?
+
+**Finding**: The `driving_path_flag` is stored at the ACTIVITY level on the TASK table, not on individual relationships.
+
+**Evidence**:
+```
+TASK table includes: driving_path_flag (Y/N)
+```
+
+This flag indicates whether an activity is on the driving/critical path, but does not indicate which specific predecessor relationship is driving that activity's dates.
+
+### Question: Can driving relationships be derived from available data?
+
+**Finding**: Yes, driving relationships can be computed using schedule date comparison logic.
+
+A relationship is "driving" when the successor activity's early start is constrained by the predecessor's completion. For a Finish-to-Start (FS) relationship:
+
+```
+driving = (predecessor.early_end_date + lag_hours ≈ successor.early_start_date)
+```
+
+### Decision: Compute driving flag at query time using early dates
+
+**Rationale**:
+1. P6 does not export a pre-computed driving flag per relationship
+2. The driving relationship determination can be computed from activity dates
+3. This matches how P6 itself determines driving relationships in the UI
+
+**Implementation Approach**:
+1. Early dates (`early_start_date`, `early_end_date`) are already parsed from TASK table
+2. When querying relationships, compute `driving` by comparing dates
+3. For FS: Compare `pred.early_end_date + lag` to `succ.early_start_date`
+4. Use 1-hour tolerance for floating point date arithmetic
+
+**Alternatives Considered**:
+1. **Static flag from XER**: Not available in standard exports
+2. **Always false**: Would not provide value to users
+3. **Require user to specify**: Adds complexity, not aligned with P6 behavior
+
+### Schema Impact
+
+No schema changes needed for relationships table. Required activity date columns are already present:
+- `activities.early_start_date` - Already in schema ✓
+- `activities.early_end_date` - Already in schema ✓
+
+The driving flag will be computed at query time via JOIN on activity dates.
+
+### Validation Plan
+
+- [ ] Verify early_start_date and early_end_date are parsed correctly from TASK table
+- [ ] Test driving computation against known P6 schedules
+- [ ] Confirm results match P6 "show driving" feature where possible