Stabilize process cancellation tests

[mattfaltyn]

Summary

• wait for PID marker files to contain a parseable PID instead of treating file creation as complete synchronization
• apply the same synchronization to real-signal and fake-Git cancellation tests

Why

Protected-main dogfood run 30293089977 exposed a race where shell redirection created an empty marker momentarily. Product behavior passed; the test parsed the file before the shell wrote the PID.

Validation

• go test -race -count=10 ./internal/app ./internal/repo ./internal/process
• go test -race ./...
• go vet ./...
• go build -trimpath ./cmd/intentci
• go run ./cmd/intentci --all

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