Rewrite IntentCI as lean Mac-first v2

[mattfaltyn]

Summary

• replace the v1 compiler/evidence platform with a strict changed-file-aware command runner
• reduce the runtime to 520 production lines, 533 test lines, and two direct dependencies
• ship one Apple Silicon macOS workflow and one v2.0.0 release archive
• preserve v1.1.1 through immutable tags and releases with explicit migration guidance

Validation

• go test -race ./...
• go vet ./...
• go build -trimpath ./cmd/intentci
• go run ./cmd/intentci --all
• retained Go example tests
• native packaged archive checksum, version, init, changed-file, and --all smoke tests