SMITH-020 Implement Source Control and task presentations

[webfish0]

Outcome

Developers can inspect changes and run build workflows.

Scope

• Reuse SCM models for repositories, groups, resources, staging, and commit input.
• Diff navigation and SCM status decorations.
• Task selection, dependencies, lifecycle, output, cancellation, and problem matchers.
• Workspace-trust gating.

Acceptance criteria

• T-013, T-016, and T-023 pass for SCM/task fixtures.

Requirements: FR-010, FR-014, US-003, US-005, US-009

Source: backlog.md SMITH-020.