Step 5: wire git-impact context/prompt/source checks

[siisee11]

Summary

• Implements Step 5 for git-impact: CLI arg wiring into structured analysis context, initial prompt generation, and check-sources source validation flow.
• Adds command surface updates in Cobra (--config, --output, analyze flags, and check-sources command).
• Moves the Step 5 execution plan from docs/exec-plans/active/ to docs/exec-plans/completed/.

Milestones completed

• M1 Baseline/scaffolding alignment completed.
• M2 Context and initial prompt implementation completed.
• M3 CLI command surface wiring completed.
• M4 Source check implementation plus JSON/text output modes completed.
• M5 Test coverage and verification completed.

Key decisions

• Follow SPEC.md section 4.3 and section 7 as canonical behavior for source discovery and CLI/context interfaces.
• Keep automation-facing output machine-readable via --output json, with human-readable text mode.
• Use explicit optional analysis selectors (pr, feature) in AnalysisContext while preserving since and config provenance.
• Detect provider families with case-insensitive provider_type matching for GitHub/analytics sources.
• Use a VelenClient interface plus CLI-backed implementation for deterministic tests and spec-aligned runtime behavior.

Test plan

• go build ./...
• go test ./...
• Unit coverage includes context/prompt generation and check_sources success/failure/capability and call-order behavior.