feat(people): build fct_staff_attrition

[cristinabaldor]

Summary

Implement fct_staff_attrition as a periodic snapshot fact table capturing each staff member's attrition status at the close of each academic year, across three measurement methodologies (foundation, nj_compliance, recruitment).

This replaces fct_staff_attrition and fct_staff_terminations from the prototype marts folder and plugs into the star schema defined in the 2026-03-27 design doc.

Design

See docs/superpowers/specs/2026-04-02-fct-staff-attrition-design.md.

Key decisions

• Single model: combines attrition + termination context (fct_staff_terminations dropped — no downstream consumers)
• Grain: employee × academic_year × attrition_type (3 rows per person per year)
• snapshot_date: per-methodology window close for retained staff; termination effective date for attritors
• Source: int_people__staff_roster_history
• Location: models/marts/facts/fct_staff_attrition.sql

Acceptance criteria

• fct_staff_attrition.sql built from int_people__staff_roster_history
• Surrogate PK: employee_number + academic_year + attrition_type
• FKs to dim_staff and dim_staff_work_assignments (SCD2, resolved on snapshot_date)
• All three attrition types present with correct cohort window logic
• Interns with Internship Ended excluded from all cohorts
• Artifact termination reasons excluded
• fct_staff_terminations removed
• dbt tests pass