feat: scaffold git-impact CLI (step 1)

[siisee11]

Summary

• Scaffolded the new git-impact CLI surface with thin Cobra entrypoint wiring and analyze / check-sources command stubs.
• Added internal/gitimpact foundations for types, config loading, and analysis-context construction.
• Added repo-root ./git-impact shim and initial tests for config, context, and command stub behavior.
• Moved the Step 1 execution plan from docs/exec-plans/active/ to docs/exec-plans/completed/.

Milestones completed

• M1: Added cmd/git-impact/main.go thin entrypoint into internal/gitimpact with command stubs.
• M2: Created internal/gitimpact/types.go with initial domain/config/result structs.
• M3: Implemented internal/gitimpact/config.go using Viper for impact-analyzer.yaml load/decode with defaults.
• M4: Implemented internal/gitimpact/context.go for validated AnalysisContext construction.
• M5: Added repo-root ./git-impact executable shim.
• M6: Updated go.mod/go.sum for Cobra + Viper and added scaffold tests.
• M7: Validated baseline build/test success.

Key decisions

• Kept cmd/git-impact thin and moved behavior into internal/gitimpact per architecture boundaries.
• Intentionally left analyze and check-sources runtime logic as explicit stubs that return not-implemented errors after config/context validation.
• Encoded default analysis windows in config-loading path for predictable behavior without full user config.
• Resolved relative config paths against caller working directory during context construction.

Test plan

• GOCACHE=/tmp/go-build-cache go test ./...
• GOCACHE=/tmp/go-build-cache go build ./...