feat(gitimpact): implement WTL engine and phased delivery policy

[siisee11]

Summary

• Implemented the git-impact WTL execution engine and phased delivery policy across Source Check, Collect, Link, Score, and Report.
• Added observer lifecycle hooks and wait/resume handling via callback.
• Added deterministic tests for phase progression, retry exhaustion, and wait handling.
• Moved the step 3 execution plan from docs/exec-plans/active/ to docs/exec-plans/completed/.

Milestones completed

• M1: Define gitimpact engine contract (internal/gitimpact/engine.go).
• M2: Implement phased-delivery loop mechanics in Engine.Run (ordered progression, continue/retry, wait/resume, completion/exhaustion).
• M3: Add observer integration surface (internal/gitimpact/observer.go).
• M4: Add core control-flow tests (internal/gitimpact/engine_test.go).
• M5: Verify repository health with go build ./... and go test ./....

Key decisions

• Reused the baseline internal/wtl engine/policy loop pattern while specializing directives and phases for git-impact.
• Kept wait handling callback-driven (WaitHandler) to support future CLI/TUI adapters without engine rewrites.
• Used an explicit ordered phase list for auditable progression.
• Applied phase-local retry handling with a fixed max retry count of 3.
• Emitted observer lifecycle callbacks directly from engine run transitions.

Test plan

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