fix: allow filter-only forget runs

[rmanibus]

Summary

• allow forget policy mode to run with filters only, without requiring keep flags
• validate that snapshot_id is not combined with retention or filter flags
• add focused CLI and engine test coverage for the new argument handling

Testing

• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./cmd/cloudstic -run TestRunForget|TestValidateForgetArgs
• env GOCACHE=/tmp/cloudstic-gocache GOMODCACHE=/tmp/cloudstic-gomodcache go test -count=1 ./internal/engine -run Test.*Forget

[codecov]

Codecov Report

❌ Patch coverage is 72.34043% with 13 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/cmd_forget.go	71.11%	7 Missing and 6 partials ⚠️

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