Harden interruption and prepare v2.0.3

[mattfaltyn]

Summary

• preserve the changed-file, strict CLI, Maven/Gradle, and documentation fixes from c3d4de2
• terminate IntentCI-owned Git and check process groups with a one-second graceful shutdown before forced cleanup
• add complete init goldens, real-signal E2E coverage, and Git cancellation coverage
• prepare v2.0.3 with one source-version constant and a tag/version publication gate

Validation

• go test -race ./...
• go test -race -count=5 ./internal/process ./internal/repo ./internal/app
• go vet ./...
• go build -trimpath ./cmd/intentci
• go run ./cmd/intentci --all
• go test ./... (examples/go)
• go mod tidy (no manifest diff)
• workflow and configuration YAML parsing
• local darwin/arm64 archive checksum, contents, version, init, changed-file, and --all smoke tests

No new dependency; production code is 649 lines and still Mac-only.

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