feat(skills): admin skills management parity (#58)

[mpge]

Summary

• Adds SkillRoutingTag / SkillRoutingDepartment pivots plus AgentSkill surrogate key, int proficiency (1–5), timestamps, and check constraint wired through shared EscalatedModelConfiguration plus handcrafted SkillManagementParity migration (SQLite / PostgreSQL / SQL Server agent_skill reshaping with legacy string backfill).
• Introduces /support/admin/skills admin JSON endpoints (transactions on writes) and refactors SkillRoutingService to require agents to cover every matched routed skill while ranking by summed proficiency then open workload.
• Adds tests for admin skills endpoints, routing, and legacy proficiency string mapping aligned with migration backfill rules.

Test plan

• dotnet build (not run here — dotnet CLI unavailable in this environment).
• dotnet test
• dotnet format --verify-no-changes

Closes escalated-dotnet#58.

[mpge]

Reviewed: skills parity for #58. Cursor delivered the explicit-mapping routing tables, proficiency migrated from string enum (beginner|intermediate|expert) to int 1..5 with the LegacySkillProficiency lookup for backfill, transactional admin controller, and full test coverage.

Manual fixes applied during review:

• rebase onto current main (EF Core 9.0.16) and bump the new Microsoft.EntityFrameworkCore.Design dep to 9.0.16
• add missing closing brace on SkillManagementParity.cs
• add using Microsoft.EntityFrameworkCore.Infrastructure; so the [DbContext] attribute resolves

CI green: lint + test both pass on the latest commit. Merging.