Step 9: implement git-impact analysis progress TUI

[siisee11]

Summary

Implements Step 9 of 10 for git-impact analyze by delivering the analysis-progress TUI, wiring it into the runtime engine loop, adding default engine construction helpers, and completing plan bookkeeping by moving the Step 9 execution plan to docs/exec-plans/completed/.

Milestones completed

• M1: Expanded AnalysisModel with required progress/view state and phase-aware transitions for bridge messages.
• M2: Implemented SPEC-aligned View() layout (title, separator, progress/turn details, phase rows, wait block).
• M3: Added tui_model_test.go coverage for message handling and spinner tick behavior.
• M4: Added default engine wiring via NewDefaultEngine(...) and DefaultHandlers() registration.
• M5: Updated cmd/git-impact/main.go analyze flow for interactive TUI mode and non-interactive JSON/piped execution.
• Plan closure: moved Step 9 exec plan from active to completed.

Key decisions

• Implemented production behavior directly for this step (not a mock/prototype).
• Kept phase progression source-of-truth in engine observer messages.
• Centralized default handler registration via NewDefaultEngine to avoid duplicated wiring.
• Preserved machine-readable JSON mode without rendering TUI output.
• Limited interactive progress rendering to terminal text output.

Test plan

• go build ./...
• go test ./...